Why am I obsessed with my best friend?

Me and my best friend have been friends for a few years now however we stopped being friends and only became friends again after this summer, but we did get very close very quickly again. I find myself getting sad if he isn’t coming out when I’m with my other friends and I hate to say it but anything he does I will do as well. I want him to value me the most out of all of his friends but he has other best friends as well, which I’m not mad about I just want to be his favourite. I always get really scared that he is going to slowly get bored of me and forget about me soon, if this happens I don’t know what I would do because he means so much to me I don’t really tell him that though because I don’t want to be overwhelming. I always get scared he’s ignoring me and I just wanna know, how did I become so obsessed with my best friend?!?! Our friendship never used to be like this.