When you try to reach out to someone, but you can't.
On a similar note, trying to reach out to someone and simply being ignored.
Friends who don't tell you about good things that are happening in their lives, that they would normally tell you (the very first person, in fact), even if their intentions were good. The thought was very nice, but that's no reason to not let me be happy for you, you know?
When people are passive-aggressive in such a way that you can't tell if they mean to be that way or not. And it's not like I'm unfamiliar with the theory, either.
That point where things hit a bit of a brick wall, where circumstances force you to just grow up and move on. (lost my planned roommate)