I've always had a fascination for astronomy, and I've always been very good in my math classes, so I figured that I should find a career that combined both of them, and aerospace engineering seemed like an ideal option for me.
One benefit I see for majoring in mechanical engineering is that (I hope) it'll give me more options of which career or job I might be able to get, and let's face it, in this economy people will take a job anywhere they can get one. (After all, I've failed to get a summer job this year for the second time in a row.)