I have said this before in other threads, but man, being an introverted, singleplayer only focused kid sucked for a while, when many games had 'mandatory' co-op elements, and some even revolved around co-op as a whole.
Lots of games did this. Some of the latter Splinter Cells and Dead Space 3 (which admittedly already sucked) come to mind. I believe this was also the gimmick that Red Alert 3 had, though at least that had an AI when you played alone.
It felt like ass to reach a certain point in the game only to be locked out of certain paths and content with a big prompt that says CO-OP ONLY. (Sure, most of the time it was minor, and basically didn't even change the story, but still, it was irritating to me)