I'm a really quiet person, and I absolutely [b]HATE[/b] it when people ask me why I'm so quiet.
I mean I'm not especially mad at them as a person, as human beings are naturally social creatures, and someone who's not social is a bit out of place...but still, if they used a bit of common sense, they'd find it's a bit too much of a personal question to ask.
The reason behind it could be simple, or it could be really complicated. It could be as simple as them not liking to talk and preferring to be alone, which makes for a simple response...or it could be something deep and complicated they've had since childhood, and are trying to work out with a therapist, in which is a lot of explain to a mere acquaintance.
For my case, it's kind of both. Ever since I was a child I wanted to socialize but was afraid to do so, and I've tried to work it out but not massively improved. But more recently, I'm not super shy anymore, and it feels as I'm just not very creative and don't have very much to say...so that's my response to anyone who asks: "I'm not very creative and can't think of very many topics." But the [b]ACTUAL[/b] reason is a lot deeper, as I've been trying to work this out since childhood but no luck.