Even though I've shuffled around playing with IRL friends, and other friends I met online, I've pretty much stuck to WoW, League, with some Guild Wars, Guild Wars 2 and RS thrown in there. All of these games, despite being old evolve over time. It feels like there is always something new to do, or new to learn. I like this because I'm still playing the games I'm familiar with, but there's added content that I have to pick up, so the games don't ever truly feel stagnant.
These games also feel different depending on who I'm playing with. Take League for example. With certain friends, we try our best to play at the top of our game, mimicking what the pros do. Take the same game with a different group of friends, or a different mindset, and we queue up into normal games and try to come up with cheesey team compositions.
Same can be said about WoW. On one hand, I can have a Guild with a large community of in-game friends that I've met over the years. We do things ranging from farming achievements to downing heroic raids. I can flip my focus in the game, and then run PvP arenas with IRL friends. Even though it's the same exact game, there are so many things to do to keep it from getting dull.
Don't get me wrong, I do enjoy single player games, I'm just not one to go out of my way to pick them up. If you're more of a story-driven single player game type of guy I can understand the dilemma about finding games to play. Even the most well-made games can be explored and completed 100% with a few months of dedication.