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.