Why does this keep happening

Hello, today I will be bothering you about a problem of mine lol. There is a girl that is just my friend, we have 'dated' for a little while a while back though. I say 'dated' because really it wasnt even anything different than normal. Anyways, we are all in a little group of friends and we all love each other as friends and we say that. I really love her as a friend, but there is a bad cycle in her life, she gets happier and more loving and all that over a time period of like 8 months, and then all of a sudden, she just completely like, retracts from life and everyone and "doesn't love" and all that, and it just feels like everything was a lie. Then it starts all over again. Im not trying to "get her" or anything like that, im 15, dating and all that is just a stupid idea. I just want to be friends and for things to be stable, and I love her, and she 'loves me forever' but then she doesnt. It makes me feel as if it's just a terrible idea to have friend love or something like that. Has anyone ever experienced people like this, or better yet, do you know firsthand how it is? I dont know what to do anymore

I certainly find it quite odd, that your group has this fixation on using the word "love" a lot to describe a friendship. There is nothing wrong with that, just normally people don't throw love around in that context casually. I don't feel her "love" was faked or a lie as you put it. It'd make no sense for her to act that way for 8 months then suddenly not. Plus she hangs around you with, and you "dated". So there is clearly something there that isn't just playing along with being all happy. The reason she's like this may be because of a few factors. If she has this periods where she acts this way it may be because of some life event that occurs round that time? Something that perhaps makes her stressed, like exams? Some family tradition? Some event? Or job she part takes in? You get the idea. Another reason could be age, girls experience puberty in the ages between 10-15. Which she is 15 and may still be doing through it, hence hormones can do some crazy stuff, such as you making you this way. All bipolar, at certain points in life, during puberty you get random hormone bursts. That could be something to do with it. But if happens currently next year perhaps it's probably more likely some life event that gets her stressed, and therefore easily irritated by people. I hope that helped, I hope you have a wonderful day :)