Think about it. That person you want to say hi to might have had a really bad day, and a simple hi might lighten it up. Throw in a small conversation, and maybe a cheesy joke, and you might get that person to smile! That could mean a lot to him/her. And you'll boost your self-confidence as well

You don't have anything to lose. If you somehow get rejected, which I don't think you will, you never knew the person anyway, so it doesn't matter ^^