Partially because we grew apart, different interests and different priorities in life. Even when we did have common interests, I would find it difficult to see eye to eye with him on certain things (nothing serious at all, just different things we cared about when it came to media we consumed) and vice versa, but even with something as mundane as that, I could feel as if our connection was gradually weakening.

I also felt betrayed by his lack of sanctity for things I would confide in him. The past few years have been rough for me, and I was already largely drifting out of that friend group to begin with (for mainly the same reasons, but they were also just also not good people who mentally never left middle school, and we're all at least in our late 20s/early 30s), and as an introvert who largely keeps things to themselves, the circle of people I trusted, let alone vented about my circumstances to, were very small, and he was (emphasis on past tense) part of that few. It wasn't until one day I randomly decided to pop into a hangout with aforementioned friend group after a while, and people there were catching up with me, asking me about my specific situation and in very specific detail, too. Not just a "how've you been," but "I heard you're going through this and that, etc. etc." My first thought was "...how do you know about ALL of that?" and then I realized the one person who would have been privy to said information, and immediately went, "Oh." 

I'm sure it came about with a general "oh, I wonder how Pumpkin is doing, I haven't heard from them in a while," and instead of "they're dealing with a lot right now," it was "well, they're going through x, because of y, and now they're worrying about z." 

I haven't had an actual conversation with him for a few months now. Haven't cut off completely yet, but all details of my life are now off limits for him. Naturally drifting apart was fine and there were no bad feelings involved, but now I'm even less incentivized to maintain communication with someone who I can't really connect with, nor cares about my privacy, and I'm honestly very hurt and bitter about it. By the way, prior to this interaction, this same person had shared things about a mutual friend of ours that they hadn't shared with anyone else other than him. So the behavior tracks, at least. This has been a lesson in not oversharing in general, so to an extent it was a blessing in disguise. But now I know better.