Guys in HIGHSCHOOL please read this? and ill answer your question too?

Okay well me and my ex have been broken up for about a year and a half now and thats a pretty long time. during this time he basically acted the opposite of how when we were going out : he was always rude to me, he teased me a lot and he kinda kept his distance but not to much where we wouldnt keep in touch. we were still good friends even after we broke up cuz we would hang out all the time and we texted a lot. but during this time he also told me that he loved another girl and that he couldnt stop loving Her. but recently we just had a water balloon fight and he was i guess near me a lot and we were having a lot of fun and then the next day we hung out again and my friend said he just gave me a look that was like "i want you" . later he just told me that he thought i looked amazing and beautiful then he started being nice to me and he was acting like how we were when we were together. but it was just so random ! but then a few days later we went to some party and we were having fun and stuff and he was flirting and everything but then people were making fun of us saying that we were basically like a couple in love . and ever since that day he has been acting the same like how he used to be which is basically rude and not caring. so WHY DID HIS EMOTIONS CHANGE SO QUICKLY??