Why do people keep thinking my cousin and I are boyfriend girlfriend?

We're really close, and he's the closest thing I have to an older brother (and I've always wanted one). So I think it was Monday last week, my last day in the Philippines, visiting my family. My whole family on my mom's side went to Mann Hann for lunch. After lunch my mom and her sisters walked around the mall just to chat for a while before we left. My younger cousins went to one of those play places you can find at the mall so my only older cousin and I were left. My mom said me and my older cousin were old enough to walk around alone. We went to the arcade and played Tekken 6 mostly. A while after I left the Philippines my cousin and I were chatting on facebook and he said he had something to tell me. He said one of his classmates asked if he had a girlfriend. I thought for a sec that she liked him and that's why she asked but then he said that it was because she saw us at the arcade, and thought I was his girlfriend. Then another time, my cousin and I were on a post thingy on this site (Dollars), and I was just telling him something. Earlier today he told me that one of the people on the post that was there thought I was his girlfriend... and my friend one time, came over and saw a collage of pictures we have hung up, and there's one of me and my cousin, and she asked, "Is that your boyfriend in the Philippines?" Even my mom said that in that picture my cousin and I looked like boyfriend and girlfriend! I don't see it though... I mean when we were at the arcade we didn't hug or anything like that at all, and on the post thingy I was just asking him about the storm in the Philippines right now. We're 4 years apart by the way, and we treat each other like brother/sister because hes like my brother and im like his sister... i think that was pretty obvious though ^_^