The opposite of love isn't hatred or anger, it's apathy.
People often rant about how they are leaving games because they are jilted lovers.
Agreed!
In any forum, there is status. People feel ownership here. They feel like they have a presence, a reputation to uphold, a virtual form of currency that they need to maintain.
Switching a game that they heavily identify with (and being hardcore gamers who post about gaming in their free time, that's us) is a significant switch. So it's only natural that people post about it.
What's different from this hobby than so many others is that the rules you choose to play can actually alienate you from other players. This isn't fair, and it shouldn't be this way, but it can certainly happen. The problem is that often, choosing to not play a particular rules system means losing social status by voluntarily withdrawing from a group.
This pushes people. If you're out of a game, that player is out -- seriously, both feet, out of a game. They may be leaving a peer group as a result. Some players might feel passionate enough to share their rants about exiting systems with their peer group, but it's likely they don't get the chance. Forums give them that chance.
This is the price we pay for our hobby being almost entirely focused on the social nature of people getting together day after day, week after week, to play the same game. It takes commitment, and one does not walk away from that commitment lightly.
Incidentally, I really don't think it's as apocalyptic as all that, but for many gamers it SEEMS that way when they leave the peer group they know. There are always other groups out there and other games...