I have lost interest with MMA, gained it again, lost it again, gained it again, quite a few times. It just depends on how good the fights are at the time, who the champions are (thus how interesting the title fights might be), how the divisions stack up at the time, etc.
Right now the sport is a bit more interesting because Anderson/GSP are not champions anymore. That's two divisions that open up for new challengers and interesting title fights. I used to watch every event from UFC to Pride to SF, WEC, etc, but then I got too busy and lost interest in keeping up with all of that, started to focus on just the UFC (especially after they bought almost all their competition), and now my interest sways in the wind from interested to not interested.