Because I like flying? The freedom, the abilty to get to places and back in a day that would be impossible in a car, the challenge and satisfaction of a flight well done, the views, the pleasure of breaking out of an overcast into a gin clear day with clouds like snow beneath you, being privileged enough to have the ability to do something that not many others do. The list is endless.
Bit like asking why someone goes fishing. Probably because they like going fishing...
Edit: Just read that through and it sounded a bit harsh, have put a smiley on the end.
There's another one I've just thought of as well that's important to me, the camaraderie of fellow pilots. Others may disagree but wherever I fly to I always get a good welcome and am talking rubbish (just like on this forum then!) about flying to a group of strangers who are talking rubbish about flying within five minutes.