A lot of things really but one main issue.
Why is it when someone criticizes us, or gossips about us in a derogatory manner, or maybe they just don't think too highly of us, why do we assume everyone feels that way about us? Why is everything that comes out of our mouth suddenly suspect? Why do we obsess over words and actions?
Why do we let a few people dictate our behavior? Why is it suddenly of utmost importance, bordering on a National Security issue to us?
I'd like to think it is because we are all such lovely people and we can't imagine someone snubbing us. But the gut truth is, we aren't. I'm certainly not. I'd love to be but I'm not. I have issues just like everyone else, just like you. Some of your issues might be a bit more "glaring" than mine, but that gives me no right to criticize you. Especially not to other people.
It's been said before and to say it again would be to beat a dead horse, but I'm going to say it anyway. If you can't say something nice, say nothing at all. The saying does not say, "If you can't say something nice, lie through your teeth about them." If you think someone is slighting you, go to them. Don't go to everyone else about how mistreated you are. Go to the person and talk about it.