It is probably because you don't really have a planned out reason yourself right now. Often it's uncomfortable to talk about the 'why" when one doesn't have a clear answer to that themselves. The simple answer "can" be, "It's because it is something I have wanted to do in my life".

I think what is more important with this question is "how" it bothers you and not that others are asking this, which is actually a common question people ask by the way.