Let's see here. When I was young, my family always told me that I couldn't talk to girls, because I only ever talked to other guys. I just didn't see what the point in talking to girls. I went to a small primary school, and there were only 9 girls in my year level. None of them had any common interests with me, so there really was no point me to talk to them. Then my parents sent me to an all boys school during middle school.
When I was about 15, my cousin was talking about his girlfriend. I really didn't care. But of course me being socially inept I just flat out told him, "I don't see the point in having a girlfriend". He must have thought that I was a bit strange. This was about 3 years before I started identifying as aro.
There's also the fact that I really wouldn't even know what to do in a romantic relationship. I get really uncomfortable with physical contact (except handshakes), so much so that I can't even hug my own grandmother without feeling awkward. I can't imagine how awkward it would be if someone kept trying to be physical with me.
EDIT: I also used to really hate romance in fiction. It just seemed so utterly pointless. This has died down a bit since then. But still, shows like "the bachelor" are just the most pointless creations ever dreamt up by a human being.