Because if, at any moment, your existence could end, and what happened in between now and then is irrelevant - the end result is the same. And even if your existence has an impact on the world past your lifespan, that will eventually be gone, and none of it will have mattered - i.e. none of it will have had meaning. If you are going to end up in a certain situation, with the same exact circumstances (this means physical, mental, emotional - all possibilities converge into one) no matter what you do, does it matter what you do? No.
I can say that I am divided between Kierkegaard and Nietzsche here - though I usually lean towards Nietzsche; while humans are capable of making value in life for themselves, making that value is pointless, and therefore irrelevant. They can do it, as I am doing right now, but it really doesn't matter either way.
I have a feeling I am getting too abstract, which isn't a good thing for me...