Generally when I leave a game it can be because of a multitude of reasons. The most common of which being "The game doesn't hold my interest" And for that, there's nothing to be done. I maybe just wasn't in the right mindset to embrace a new setting or could be any number of reasons, there's no solution to be had there.
Toxic playerbase. If I feel like the people that I'm playing with are assholes, than why would I continue to play there? This is especially true when the games systems encourage exclusion and cliques, leaving newer players on the outside and unable to get a leg up on the other systems.
Lack of social features: While this isn't for everybody, if the game doesn't have a healthy amount of OOC interaction, it's just not going to be the place for me. Sometimes I like to know the player behind the screen, to talk and enjoy each other's company in a way that lets us connect as people.
Bad Staff: The most common reason really, is bad staff making unethical decisions, cheating or just basically treating the game like it's their own personal sandbox. And... that's pretty much it?