Why are my friends so immature?

so im a 15 year old guy and theres basically a group of 5 guys, me included, that do stuff together. well we all play hockey together and do almost everything together - go to movies, hang out at one of our houses, go to parties, etc. but it seems like a lot of times 2 or 3 of the people in this group will decide to do something together and not tell the others. then whenever the rest of us try to call or text the people that are doing something, they ignore us. also, they all talk about each other behind each others backs (they probably talk about me too) but then all of the sudden they act like bestfriends and do stuff without inviting anyone else. so i basically sit in my house all summer because these people dont ask me to do anything, and they hang out with the same people that they talked sh*t on the day before. also, i invite all 4 of the other people to everything i do , but then it seems like im always one of the ones who are left out.. i just dont get it they all act like theyre 2 years old