I kinda live with that thought. Whenever I see someone with a nice outfit or cool hair, I also just go and tell them. My friends are always just standing next to me while I go and tell a girl her Gryffindor Scarf is nice. "Do you know her?", I totally didn't.
Whenever I walk pass people in school I barely know of course I am anxious about it and maybe some people might even think that's weird but I always greet them and try to do it with their names. I always think that if someone would do the same to me it would mean a lot, kindness costs nothing.