I have been asking myself this question for a while now, but the debates I've had with myself haven't produced a satisfactory answer in the time that I've had to think about it. This is my attempt to put these debates into words, in the hope that it might help.
So, who or what decides how good of a motivator something is? Before we jump in to that, how do we even decide whether a motivating factor is good or bad? Can we even objectively arrive at an answer to that? The easy solution is to do away with the trouble of trying to do so, and declare it to be subjective. That doesn't satisfy me. Maybe we just haven't thought about it enough? Even if it does turn out to be subjective, it would make an interesting exercise to realize why this is so. Further, if one is going to make major decisions like choosing their career based on a motivating factor (say interest or curiosity), what is the guarantee that he/she will have the same drive ten-twenty years down the line? Is that even an important property for a motivating factor to have? Having established why this exercise is worth the time, I'll get started.
How do we decide if a motivating factor is good or bad? Are there any axes along which we can quantitatively measure the "goodness" of a motivating factor? Even if we don't actually assign values to these factors along these axes, can we get everyone to agree that a particular factor lies further along an axes than another factor? Let's being by trying to define such axes.
As I've indicated above, one important axis might be sustainability, i.e. how long will this motivating factor continue to drive me to obtain the goal it defines? Let us try to take two motivating factors as examples to see where we can place them along this axis: competition and interest. These two motivating factors have roots in different emotions, namely pride/jealousy and love/curiosity. Trained as we have been from childhood, just putting these into words brings about good feelings for interest and bad ones for competition. But we'll get to that shortly. Let's look objectively at how sustainable competition and interest are as motivators. Note that a motivator is always associated with an end goal, so if your end goal changes, the motivator has changed as well. Under this definition, competition lasts as long as you have peers to compete with. Interest is harder to limit, as it could be either because of curiosity, or because of love. Interest due to curiosity as a motivator lasts as long as you don't have enough of a flavor of that topic to decide whether to work towards a new goal (in which case the motivator changes) or not. I'm going to be hand-wavy in saying that interest due to love is not well thought out, and there is actually another driving emotion that defines why you love the subject/field/job. If we think about it, we actually love some subject/field/job because of a few reasons. Maybe it sates our thirst for puzzles, in which case the true emotion is the pleasure of satisfaction. We want our brain to release more of the chemicals associated with the feeling of satisfaction. Maybe it makes us nostalgic about something from our childhood, in which case the true emotion is the pleasure/grief associated with nostalgia. Anyhow, let us say that we break down interest due to love into one (or more) of these emotions. Let us compare, for example, motivation due to competition, curiosity and pleasure of satisfaction. How sustainable are they? Leaving out a few cases, almost everyone can always find peers to compete with. You can find ranks to grow in an organization, peers to beat or stay ahead of while publishing, stats of peers to beat as an athlete, etc. Especially if you set a vague enough end goal (like I want to rise in ranks as much as possible, independent of the organization), you can sustain that motivation for quite a long time. Remember that we are associating a motivator with the end goal, so I argue that we can always objectively place it somewhere on this axis relative to other motivators. Motivation due to curiosity is not very sustainable (as you might have figured without even performing this analysis). To put it in formal terms, you don't really know your end goal when you are curious about something. Thus, you are forced to set a short-term end goal, which is bounded by what you know about that subject. Our motivator changes once we understand enough about the topic to switch goals. Similarly for motivation due to the pleasure of satisfaction: once we are satisfied with a particular puzzle, the motivator changes. One might argue that we can pick a puzzle that will never be solved, or equivalently, will take our lifetime to solve. But can one truly find a puzzle of that form (a concrete task, and not a vague area)? Even if we do find such an area, would someone be motivated enough to pursue a puzzle that won't result in satisfaction (take an infinite amount of time to do so)? Is the true emotion that drives that person still satisfaction in such a case?
By our definition, given a 3-tuple of an (end goal, emotion, context), we can objectively argue about the sustainability of such a motivator. Note that it is no longer truly subjective as we are embedding all the relevant information into that 3-tuple. To conclude, sustainability(motivation due to competition to always grow to the highest rank) >= sustainability(curiosity to learn more about a vast area) >= sustainability(get satisfaction by solving a particular puzzle).
Let us look at another axis, daily happiness, and see how the same motivators rank. I understand that my choice of axes is arbitrary, but the point of the exercise is the following: given a set of axes and 3-tuples, can we objectively argue (agree on) where these 3-tuples lie along these axes? If you want to choose another set of axes, then by all means, please do so! But when we share the set of axes and 3-tuples with each other, we should agree on the placement of the 3-tuples.
Back to daily happiness. Motivation to always grow to the highest rank possible rank will result in very short impulses of happiness on a daily basis. This is because your highest rank is infinitely far away (by definition), and even if you progress in ranks once every few months/weeks/days, the incremental happiness would theoretically be very small. Further, since competition has its roots in pride/jealousy, watching your peers rise in ranks while you don't can be frustrating and affect your daily happiness. Since you could potentially be competing against a large number of people, and someone or the other will grow in their rank (due to ORs on probabilities of someone growing), the gains are small while the losses can potentially be large. What about daily happiness due to curiosity? Since you don't know much about the area, with high probability, you will be learning something new on a daily basis. Each one will give you more happiness than the previous case, as you are working towards a goal that can probably be achieved in a much shorter period of time. Finally, motivation due to the satisfaction of solving puzzles. This again will provide more daily happiness than the motivation due to competition, as you are making more progress towards your goal on a daily basis; Arguably more than the curiosity case as this is achievable in a much shorter duration. In short, satisfaction from puzzles >= curiosity >= competition. But does this mean the sustainability and daily happiness axes are inversely related? Can we find a counter example where a motivator (3-tuple) provides more incremental happiness than another motivator (3-tuple), while also lasting longer? I'm running short of time, so I'm not going to pursue that train of thought.
Let's do one more axis: self improvement (is that also a motivator? Can we define a 3-tuple for it?). Does competition necessarily lead to self improvement? Not in all cases. Since your goal is to improve in ranks relative to your peers, you might find shortcuts to come out on top without actually having gained anything through the exercise. What about curiosity? By definition, you are pursuing a goal to learn more about the area and how it relates to other areas. We could all agree that self improvement in doing so will be more than in the case of competition. How about satisfaction due to solving puzzles? Since your end goal is to solve the puzzle and get satisfaction out of the intermediate steps, you might learn just enough to solve that puzzle and not more than necessary (since you are trying to rapidly progress in the direction of solving the puzzle). While the gains will potentially be more than in the competition case (you need to gain some basic knowledge to solve the puzzle!), it will be less than in the case when you are driven by curiosity to learn more about a particular topic. Thus, curiosity >= puzzles >= competition.
We could define other axes, but in the interest of time, I'm going to stop here and ask, what have we learned from this exercise? We can see that given a set of axes and 3-tuples that are motivators, we can (potentially) objectively agree on where these 3-tuples should be placed, relative to each other, along each of these axes. Can we absolutely declare that one motivator is better than another? Well, can we absolutely agree about anything in that fashion? Can we argue that an algorithm is absolutely better than another one? What about a Machine Learning model? A processor? An operating system? We define axes, such as time complexity, space complexity, error on a given task, cycles to execute a particular benchmark, time efficiency of handling signals etc. and objectively argue about how these algorithms/models/devices/systems compare along these axes. Does that mean that the comparison is subjective? Not really. How these things rank against one another is objective given the axes. What is subjective is the relevance of the axes.
Similarly, I argue that we can objectively compare the "goodness" of the motivators along some given axes. What is really subjective here, is how important these axes are to us at a given point of time in our lives.