Maybe it's just the particular cohort of friends that you go out with that are more extroverted (nothing wrong with that) and more flirty in the first place casting you in some kind of shadow.

I'm not too sure why people have to be so nosey as to ask if you have someone in your life; but you're right - it's an assumption on your behalf to think there's something wrong with you.

I don't think it's your appearance. You never heard of someone say someone not that pretty can appear as the most attractive person a room because of the confidence they portray and their personality?