Losing my bestfriend?

I am a girl and my (ex) bestfriend is a boy - we have had a little bit of a history but mostly our relationship was just being bestfriends - well so I thought. I would like someone to please explain or maybe give me some insight on what might be going on on his end because I am so devistated at whats happened, I find myself becoming more and more depressed each day because of this... so here goes

*for the sake of telling the story, ill call my friend Bob

A bit of background and context:

Bob and I had been bestfriends ever since the beginning of High School, we did pretty much everything together, I would help him with his chores and stuff all the time and we would gym together and a lot else. Gym was a big thing for us, we would train hard together. We would also get up to a little mischief together as well, experimenting with recriational drugs and whatnot - on that note, I would like to make a point that Bob was the one who convinced me try MDMA.

So theres some background, now for the incident to which made me lose my bestfriend - which I would like some help understanding please...

So Bob and I would organsie to go to the gym day by day just over message or wed just pop over to one anothers house (we literally live across the road from each other so organising lifts and whatnot was super easy) In early-middle of Feburary this year (2018) we would organise to go to the gym and then Bob would cancel on me, this happened for 4-5 days in a row before he really hurt me - we organised to go to the gym and everything was fine, a couple hours later he messages me metioning that because its nice weather we should train at the beach, which i was completely fine with and went along with that plan, then about an hour after that Bob messages me and says "*your ex boyfriend* is coming", now I understand that to some people this wouldnt really be an issue but I was really hurt by my ex and i had multiple convesations with Bob, telling him how i wanted to stay away from my ex so i could get over him and all the things he put me through - so i KNEW he was cancelling and doing that on purpose, but everyone i mentioned that to said something like "no no hes probably just got stuff coming up and got really busy" - if that was truely the case then i would know about it and i would have probably been helping him but thats beside the point

anyway a couple days after that incident, Bob sends me a message saying something along the lines of - Im sorry for lying to you (this is him essentially admitting to cancelling on me etc on purpose!!), you are a bad influence on me, dont talk to me anymore.

that is a very short version of what he said but all the important bits are included - so i respected his wishes and haven't spoken to him since.

now theres a couple things that i would like explained please if possible because i cannot understand it at all.

1. How can one just kick a friend out of their life like that when I had done nothing to directly influence that (him cancelling on me happened all of a sudden)

2. I was entering the gym at the same time he was one day - very soon after all this happened and he said hi to me like nothing happened but then after theres been no communication at all, i have seen him at the gym many times since but have avoided him as im so incredibly upset to have had this happen to me

3. Today is his birthday and I've now realised the mistake i made in messaging him to wish him happy birthday - because now ive created expectations that i know arent going to be met (my birthday was fairly soon after he sent that message, and unfortunately i did expect him to message me - soon to realise my expectations were too high, also soon to realise that he just does not care, but my question is how? how can he just drop me like that and not care after sharing such a great friendship for 8 years

I apologise for blabbering on - im honestly just writing my thoughts, i know its a lot to read but would someone please help me understand whats going on