When I was young I was taught that it was rude to invite yourself to something.
I've continued to follow that - if I'm not invited I won't go, even if I know it's on.
It wasn't so bad back then - no Facebook, no instant communication (other than phone calls), so mostly I just didn't even know something had happened.
These days you get updates before, throughout and after an event from all forms of social media. So it's hard to see everyone else doing things. Not to mention the fact that I already feel completely left out and disconnected from all my peers. So when I find out that I wasn't even invited to a significant event of someone I consider to be a friend, it hurts - A LOT!!
So, let me tell you, if you want me to be there, you need to invite me!!