My mom tells me this quite frequently, especially when I start kvetching about workpseub said:2. "Life is too short to waste time on people you don't really like" (from my Grandmother).
My grandmother has an interesting saying in Spanish: "Ala la cadena." Translated into English, it literally means "flush the toilet". But the way she means it, is to just let it go. Don't dwell on it. It happened, deal with it and move on. I tend to worry a lot and overanalyze everything. Every time I she says it, I laugh because I know what it means in English, but it's so true.
I've also learned, not from anyone in particular, to just be nice to people. You'll get a lot farther in life, and you'll be much happier