Because i don't want to do anything else for a living. You only live once and might as well do what you love.
Like others have said, how do you know you love it? You haven't even flown for an airline yet! Flying around recreationally or while your training when your young and inexperienced is alot easier and fun than trying to survive in the real world of the airline biz. Unfortunately, we probably all felt the same way before too...