Thanks for the effing invite.... It's times like these when you question whether your "friends" are actually your friends. You have fun with them and hangout like buds but then they start to leave out of things and you start to question why. Straight bull. This is not a rant. This is a response for people who experience negative emotions and need to see a logical view at why you might feel like punching a wall or finding a hole to cry in or just forget real life and take your mind off of things.
The first factor is betrayal. These negative emotions derive from the initial feeling of betrayal. You trusted your friends and thought that you'd do alot of things together. Along the way, they start to cut you off. That really, really sucks. If true friends depend on each other and help one another out, how can either happen when you're kept out of the loop? Nah, those aren't friends. They're just pretenders. Ditch them while you can, find some new ones that you can actually depend on, and hopefully it'll all work out. I know sometimes friends can fail to invite you simply because they forgot, but that's actually pretty jacked up too. Think about it: they invited everyone else, but you were less important enough for them to completely overlook you. Not cool.
The second factor is wondering. Questioning their reasons for leaving you in the dark. If they say they're sorry but don't look sorry, that's pretty bad. If they give you a terrible excuse, that's worse. If they don't tell you why, call it quits. Until they explain why they left you out, you'll spend a decent amount of your time wondering why. Time that can be spent doing other things, like homework, studying, or socializing. If they can't bring themselves to tell you, they're not worth your time. You can try to talk things out after, but make sure you don't let your guard down. Don't let them win you over without considering the facts first.
The third factor is anger/depression. Your anger or depression comes from your inability to piece together a theory for why your friends left you out. As you spent your time trying to wrap your head around it, you start to get frustrated. that frustration probably will either turn into anger or depression. You'll become heated and ready to punch or kick the next thing you see. Or you'll get sad and want to hug the next object or person you see. Or you'll take the stone-cold killer approach and block out reality. Emotions can be unpredictable at times.
Life is unforgiving but you form relationships and bonds with others to help you cope and move through life. If your friends betray you, then you need to find new ones. If you want to give them another chance, I hope they don't betray you again. In the end, all you can do is keep on keeping on. Good luck, friend.