There are many reasons to ask this question. Planets come and go. Stars perish. Matter disperses, coalesces, forms into other patterns, other worlds. Nothing can be eternal.
Everything has its time and everything dies. Then again, for some people small, beautiful events are what life is all about.
Examples of why live? Edit
>NuWho didn't have a good Cyberman story yet.
>NuWho didn't have a good Ice Warriors story yet.
>tfw Leela will never dominate you.