Because God-dammit it was Mom and Dad's dream for water to be free! And if you don't sacrifice your life for your father's work and the world you were thrust into some months back your a bastard.
I replayed the game a few months ago and I couldn't help but laugh at the end.
It says you are a terrible person who should be looked down upon if you don't sacrifice yourself but if you sacrifice yourself while putting the FEV in then you're apparently not as bad as if you decided against killing yourself.
Oh Bethesda, your logic is so strange if you look too closely.