2)A warning sign for getting dumped.
3)A way to get out of a relationship while causing yourself the least amount of grief possible.
"About what John?"
"Sally... its not you... its me."
Harry: huhuhuh whatfor sexy hot girl?
Girl: Look... it's not you, it's me. I don't think this is working out.
Famously used by George in Seinfeld, he gets upset when he found out someone used his line and claims he invented it.
"What? Why? What did I do?"
"It's not you, it's me."