Note: this post is basically just summarizing some of PJ Eby's freely available writings on the topic of pain/gain motivation and presenting them in a form that's easier for the LW crowd to digest. I claim no credit for the ideas presented here, other than the credit for summarizing them.
EDIT: Note also Eby's comments and corrections to my summary at this comment.
Eby proposes that we have two different forms of motivation: positive ("gain") motivation, which drives us to do things, and negative ("pain") motivation, which drives us to avoid things. Negative motivation is a major source of akrasia and is mostly harmful for getting anything done. However, sufficiently large amounts of negative motivation can momentarily push us to do things, which frequently causes people to confuse the two.
To understand the function of negative motivation, first consider the example of having climbed to a tree to avoid a predator. There's not much you can do other than wait and hope the predator goes away, and if you move around, you risk falling out of the tree. So your brain gets flooded with signals that suppress activity and tell it to keep your body still. It is only if the predator ends up climbing up the tree that the danger becomes so acute that you're instead pushed to flee.
What does this have to do with modern-day akrasia? Back in the tribal environment, elicting the disfavor of the tribe could be a death sentence. Be cast out by the tribe, and you likely wouldn't live for long. One way to elict disfavor is to be unmasked as incompetent in some important matter, and a way to avoid such an unmasking is to simply avoid doing anything where to consequences of failure would be severe.
You might see why this would cause problems. Sometimes, when the pain level of not having done a task grows too high - like just before a deadline - it'll push you to do it. But this fools people into thinking that negative consequences alone will be a motivator, so they try to psyche themselves up by thinking about how bad it would be to fail. In truth, this is only making things worse, as an increased chance of failure will increase the negative motivation that's going on.
Negative motivation is also a reason why we might discover a productivity or self-help technique, find it useful, and then after a few successful tries stop using it - seemingly for no reason. Eby uses the terms "naturally motivated person" and "naturally struggling person" to refer to people that are more driven by positive motivation and more driven by negative motivation, respectively. For naturally struggling people, the main motivation for behavior is the need to get away from bad things. If you give them a productivity or self-help technique, they might apply it to get rid of their largest problems... and then, when the biggest source of pain is gone, they momentarily don't have anything major to flee from, so they lose their motivation to apply the technique. To keep using the technique, they'd need to have positive motivation that'd make them want to do things instead of just not wanting to do things.
In contrast to negative motivation, positive motivation is basically just doing things because you find them fun. Watching movies, playing video games, whatever. When you're in a state of positive motivation, you're trying to gain things, obtain new resources or experiences. You're entirely focused on the gain, instead of the pain. If you're playing a video game, you know that no matter how badly you lose in the game, the negative consequences are all contained in the game and don't reach to the real world. That helps your brain stay in gain mode. But if a survival override kicks in, the negative motivation will overwhelm the positive and take away much of the pleasure involved. This is a likely reason for why a hobby can stop being fun once you're doing it for a living - it stops being a simple "gain" activity with no negative consequences even if you fail, and instead becomes mixed with "pain" signals.
And now, if you’re up the tree and the tiger is down there waiting for you, does it make sense for you to start looking for a better spot to sit in… Where you’ll get better sunshine or shade or where there’s, oh, there’s some fruit over there? Should you be seeking to gain in that particular moment?
Hell no! Right? Because you don’t want to take a risk of falling or getting into a spot where the tiger can jump up and get you or anything like that. Your brain wants you to sit tight, stay put, shut up, don’t rock the boat… until the crisis is over. It wants you to sit tight. That’s the “pain brain”.
In the “pain brain” mode… this, by the way, is the main reason why people procrastinate, this is the fundamental reason why people put off doing things… because once your brain has one of these crisis overrides it will go, “Okay conserve energy: don’t do anything.”
-- PJ Eby, "Why Can't I Change?"
So how come some important situations don't push us into a state of negative motivation, even though failure might have disastrous consequences? "Naturally motivated" people rarely stop to think about the bad consequences of whatever they're doing, being too focused on what they have to gain. If they meet setbacks, they'll bounce back much faster than "naturally struggling" people. What causes the difference?
Part of the difference is probably inborn brain chemistry. Another major part, though, is your previous experiences. The emotional systems driving our behavior don't ultimately do very complex reasoning. Much of what they do is simply cache lookups. Does this experience resemble one that led to negative consequences in the past? Activate survival overrides! Since negative motivation will suppress positive motivation, it can be easier to end up in a negative state than a positive one. Furthermore, the experiences we have also shape our thought processes in general. If, early on in your life, you do things in "gain" mode that end up having traumatic consequences, you learn to avoid the "gain" mode in general. You become a "naturally struggling" person, one who will view everything through a pessimistic lens, and expect failure in every turn. You literally only perceive the bad sides in everything. A "naturally motivated" person, on the other hand, will primarily only perceive the good sides. (Needless to say, these are the endpoints in a spectrum, so it's not like you're either 100% struggling or 100% successful.)
Another of Eby's theses is that negative motivation is, for the most part, impossible to overcome via willpower. Consider the function of negative motivation as a global signal that prevents us from doing things that seem too dangerous. If we could just use willpower to override the signal at any time, that would result in a lot of people being eaten by predators and being cast out of the tribe. In order to work, a drive that blocks behavior needs to actually consistently block behavior. Therefore attempts to overcome procrastination or akrasia via willpower expenditure are fundamentally misguided. We should instead be trying to remove whatever negative motivation it is that holds us back, for otherwise we are not addressing the real root of the problem. On the other hand, if we succeed in removing the negative motivation and replacing it with positive motivation, we can make any experience as fun and enjoyable as playing a video game. (If you haven't already, do check out Eby's Instant Irresistible Motivation video for learning how to create positive motivation.)