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. Rise of the Cybermen/Age of Steel was at least passable, would have been better if not for some typical RTD preaching and ridiculousness. And World Enough and Time was almost absolutely GOAT!
>tfw Leela will never dominate you.
>The Eighth Doctor will never have a televised series of any kind (though the audios are GOAT enough)
>The Twelfth Doctor is regenerating.