Where to begin?
I left Serebiiforums for many reasons. I felt the people there were too bitter, too harsh, and the rules were way too strict. I gave up on the forum entirely after 2013 and have barely logged in since then. All my friends there left, moved on, etcetera.The forum is way too big, too, might I add.
I left Bulbagarden because I lost interest in the forum very quickly. There wasn't anywhere I wanted to post, so I just left quietly, changed my name six hundred times, and that was the end of it.
I left Petalburg Woods after I felt bitter when a friend of mine stopped coming there. I lost all my spirit to go there. I was pretty active there when it first opened and logged in every day. Now I don't even go there anymore. I just lurk maybe once a month and that's it. I lost all my desire to go there when all my friends who went there once just stopped going.
I left Pokemon Crossroads because I just couldn't get into the feel of it. I wasn't on the forum long, anyway, and I also felt kind of scared and didn't want to go there? I don't know. I just didn't want to go there and left.
I left Project Pokemon Forums because it was extremely dead there. So dead that there was just... nothing there to talk about...
I left PokeZam because that forum was dead, too, oh and that forum still exists. I can't even imagine why.
I left The Pallet Tribune because by the time I returned, there was no one left, so I was like "screw this. I'm not coming back."
I left Pokemasters because the admin there is a freaking stalker and FOUND MY FACEBOOK JUST BY THE FUCKING FONT COLOR I USED. I was very scared. That made me uncomfortable, so I left right away after that happened.
I left Pokedream forums because the community was terrible there, and everyone continuously called me a "he", and I didn't like that. I was there for quite a while, though, and then decided to leave after the forum became a VB4. It wasn't pleasant.
I left Mangafox forums because there was nothing to do there, and all the people butthurt about characters I like. I got sick of defending them to death, so I just left the forum and stuck with the website part. Then the website took down every single manga known to man, so I went to another manga website to read my stuff...lol.
I left Anime Forums Dot Com because my entire muse for forums died for a while in October and into about November. I joined it to start anew on the Internet and to get away from Pokemon forums, but I couldn't do it and left a month later.
I left Pokefarm Forums because I thought it was very boring after Pokebattlers was gone. This was around the time I had no computer because my parents took it away. But, I found a way to be active there. Then, for awhile, I left because I got extremely bored there. No one would talk to me, and then the entire place reset. So, I was like, lol, I'm not rejoining.
When I was 11, I joined this forum called Pokemon Nightmare. I "left" for awhile, and then "came back", then the forum was nuked, and then it became Solstice, blah, blah... then both went away... blah, blah... I remember I left Pokelight because I had no choice since my computer was taken away, and my school had it blocked or something.
I left every other irrelevant forum I joined that are dead and gone now. All of them are dead Pokemon forums that don't exist anymore, though, and if I remember those names, so help me. Solstice is one since it's dead and gone, but I didn't leave when it returned into its fourthcoming and then just died again, lol.
I've left this forum at least two times because the first time was because my mother took away my computer strip because "my behavior was bad", so I had no choice in the matter. My middle school had this place blocked, so I barely came here at all during the year of 2010. I came back sometime in 2011 because I lied and said "I had to do homework" and just... went on PC sometimes. I was slightly active from 2011 - 2014, but in October I lost interest in forums almost entirely. The only reason I still come here is to see what changes happen daily, to post in my blog and then send some of my friends with messages and then leave, lol.
TL;DR, I've left over 50 forums. Try me.