I hate it when this happens as well. Whenever i'm out and hear a group of people laughing, I immediately get paranoid and think they're laughing at me. It's a horrible feeling, and makes you feel so self-conscious, doesn't it?
I'd say that most times people aren't laughing at you at all, but sometimes you might get some childish people who are amusing themselves by making other people feel awkward or bad. I don't think its personal at all, I think they would do it to whoever walked past at that moment. If they were laughing at you, I don't believe for a second it is a reflection on you, it was just two immature boys being childish.