It really is about the people you play with. I play League with my 4 best friends in the world who've all gone to different colleges than me. That means every game of League I get to play with them is precious, and i've never once stopped enjoying League. Sure I don't really play ranked so I don't deal with that side of the community, but it's so much easier to have fun when you're playing with people you enjoy playing with. League is a social game, it's why I refuse to play if none of my friends are on.
The 'decline in fun' is due to a chain reaction. Once one player quits for any given reason, their friends subsequently stop enjoying the game without them and quit as well. The sad truth is that players quitting League is inevitable due to how long of a lifetime it has. It's kind of amazing that we spend so much time one map trying to reach the exact same goal every time, and it only gets boring to us after thousands of games. I've been lucky to have my group of friends stay together, but I understand how the game loses a lot of its magic when you don't play with friends.
So my suggestion? Go make friends! I know it sounds cheesy and stupid, but it may be just the thing that reignites your spark.