Usually, my regular groups (in which I am the DM) are all made up of friends that are friends from outside gaming. Some players have left for various reasons, mostly it's really not their thing anymore or some get annoyed by one of my other players who has a tendency to be the loudest in the room. It's never been a big deal, they retire their character, stop playing, and we all continue be friends in real life.
The only time I left a group in which I was a player was the one time that I joined a group of total strangers who had advertised for players. I had graduated from college and most of my friends were not living nearby so I was jonesing for a group. The DM was cool enough, and it seems like he would've run a great game with a different group, but the other players were immature; farting at the table was commonplace, and a few of the players were more concerned with their characters getting laid than anything else. I think I played three sessions but quickly realized that I was spending the time waiting for it to end. I called the DM and thanked him for his game, but told him that I didn't really have time to participate anymore, citing that it was taking more time than I expected away from me prepping stuff for the game I was running for my regular group (which wasn't really that true). Anyhow, I'm not sure whether or not he realized that it was his friends that were the real reason, but he was cool about it and we parted on good terms, though I've had no interaction with him since. I also cited the long commute as part of the issue; it wouldn't have been a deal-breaker otherwise, but it definitely was a contributing factor.