Well now, really, why do you feel compelled to answer such a personal question?
If there was actually a short answer don't you think that it follows that there is a short cure? There isn't, and listing the reasons will only bring more unhelpful conversation and ideas of ways to "fix you". Do you want that?

I don't answer this, never have, never will. It is not anyones business and it doesn't show anything other than their poor manners, IMHO.

If I do run across the occasional persistent person, I Always turn it around with, "I can't believe you asked me that!" and then, I change the subject. This is my personal struggle, not a topic for conversation.