Why do you like to work on some things and not others? Even in the areas that you love, why do you sometimes not want to work at all? What determines all of this motivation and drive?

One interesting way to look at motivation is to divide it into two categories: intrinsic and extrinsic.

Extrinsic motivation is the obvious one — a person is motivated to do something by an outside force or circumstance, like being paid to do a job. Intrinsic motivation is a motivator coming from within: you want to do a thing because it makes you feel good or you believe it is a good thing to do. There's no sharp line between these two things, most of our motivations are a complicated mix of both, but there's a nice and easy way to distinguish between the two. Are you doing the thing and happy about it, you want to be doing it? That signals a high degree of intrinsic motivation. Are you doing something and not happy about it? Chances are you're looking at extrinsic motivation or potentially

Motivation (at a Time)  = Pleasure (immediate) - Discomfort (immediate) + Belief * Projected Reward (at outcome) - Belief * Projected Risk (at outcome)

By this equation, being intrinsically motivated requires one of two things to happen:

Immediate Intrinsic Motivation — the immediate level of pleasure is high enough that you're motivated to act purely for instant gratification. Generally, this means we're talking about a true passion or hedonistic endeavor and the long-term risk isn't so high that it counteracts the instant reward. Note that in many cases (and for some people) it's common and easy to discount future risks in order to enjoy an immediate benefit.

Projected Intrinsic Motivation — the projected future reward is greater than the any immediate discomfort and you believe strongly enough in that projected reward that you are motivated to act now regardless of the any difficulties involved in doing so immediately.

Risk-based motivation (fear) vs Reward-based motivation (hope)

It's also helpful to divide motivation into another two component parts: present motivation and future motivation. Present motivation is, for the most part, fixed. A thing is either pleasurable or painful based on the real circumstances and your past, learned history. It is possible to change the current-time pleasurability of an action by learning a new response to that action but the only way that is done is using future rewards to entice current behavior change enough times until a pleasurable response is learned for the present behavior.

All of this is to say: the "future projections" component of motivation is much more variable and dependent on our whims and beliefs and probably most efforts at change should be focused on that section of the equation.

Counterpoint: it is probably possible, through mindful engagement and focus, to retrain your response to a present situation (to make it more or less rewarding). But, note, that dedicating that level of attention to the problem requires you first to believe that doing so will help solve and adjust your motivations in the present as desired.

Alternate titles:
Extrinsic motivation becomes intrinsic if you believe in the outcome
All motivation is extrinsic at some level
All intrinsic motivation starts externally somewhere
Most intrinsic motivation is actually social motivation
Almost all Intrinsic Motivation has a Social Foundation