Let's see, why would any sane person actually want to have...
-Hair (on head)? It needs to be washed regularly, cut regularly, lice can grow in it, adds weight to head, it can get twisted or tangled or get gum stuck in it. It'd be much more practical to be bald.
-Facial hair? See above, except it needs to be shaved/groomed more often than the head (depending on style of course).
-Testicles? The pain these can cause is like nothing on earth. And if you say "reproductive reasons" then one can ask, "Why do you need your own bio children? There's children all over the world that desperately need parents." Also, like milk, male hormones are available from other sources.
Emotions? Okay I think I made my point.
If practicality was the sole purpose of anything, rocks would be the ideal. They just sit there, existing, day to day, totally lazy doing nothing. Instead we're living, breathing, dynamic, social, feeling creatures.
Short version... um, I love having my boobs.

Kinda in the way I love laughing with my friends or listening to music or gazing up at the stars on a clear summer night.