I've played lots of co-op games over time, and almost exclusively with three people: mom in puzzle games, dad in some old NES / Genesis games he took a liking to, and my best friend. A few people in between, of course, but mostly those three.
With mom, it was casual. With dad, it was mutual. But with my best friend, who I had almost twenty years of playing alongside off and on, it was supportive. He was always acting without thinking in games, trying for whatever looked the coolest or made the biggest numbers or effects. I just naturally tried to compliment that with a support or protective role of some kind, to avoid him getting frustrated. Hell, I figured out I could do long division in my head at six years old because of helping him. And that basically triggered a joy of enjoying working out the mechanics of things for others.
PSO was a natural fit for us, and the first time it felt like I could actually fully take an active support role that still holds its own. His HUcaseal was strong, of course, but he loved daggers and twin swords to no end, even to the point of constantly being surrounded. So, I figured out how to support with the arguably best healing / casting class I could read a description of. FOnewearl for life, baby. Even after getting well past him, the strong casting nature and self-sustainability I had with my Gamecube character was fun. Offline, only eight real dungeons to run, and yet I still clocked maybe the fourth or fifth most hours of any game I've ever had on PSO.
PSO: Pleasure Supporting Others.