Well aside from all of the potential "afterlife" stuff, there's two main things that keep me going.

First, people need people to make things less miserable. Take the trick with the glasses and the popsicle sticks. A single stick wouldn't be able to stand alone. But with three sticks, they can hold one another up using the glasses as support. The most miserable man ever will be the last man to live. By removing yourself from the equation, you remove your potential to do good - no matter how small that good is.

Second, the mere achievement of it. Let's face it, the world IS a horrible place on average. But guess what? At the end of your lifespan, you will be able to look back at your life and bask in the glory of the fact that you lived that long in such harsh circumstances. Not that it's any comfort, but still...

But the final word, really, is that the world IS miserable on average - but one's existence can be used to make sure that in two, three generations people won't have to post this sort of post because the world maybe won't be such a bad place to be.

Remember - the wonderful thing about time is that it allows you to change things. Don't cut your time short.