Steven Snow from Riot Games when asked "why co-op":

"Because playing with your friends is awesome.  You know it seems kind of obvious in one sense, but I don’t think a lot of people realize it.  Look at all the MMOs that are radically popular.  Granted there are some that are totally PvP centric, but ultimately it’s about people playing with their friends.  I don’t wake up in the morning and say, 'hey I want to go beat up my buddy'… I say 'hey, let’s go play some games.'  And when we go to play games it’s not like 'hey, let me shoot you in the head a bunch of times,' it’s 'hey, let’s shoot these other guys in the head a bunch of times.' " - Steven Snow, Vice President of Productions for Riot Games, League of Legends

Bioware on why Mass Effect 2 doesn't have co-op, yet how they recognize its importance.

"We consider all features, and we recognize at Bioware recognize the benefits and the fun [of co-op]. Most of us are co-op players ourselves.  We felt that if we could do it, and do it really really well, it would be in the game.  We had to look at where we've taken the direction of the game so far and how [co-op] would affect things.  We didn't want to just do it sort of good, people would recognize that it's just a bullet point."  - Mac Walters - Lead Writer for Mass Effect 2

"Co-op is awesome, man! It offers a whole new level to the game where it’s not just about you beating the game and going through and killing enemies.  It’s also about the relationship between you and your buddy on the couch or over Xbox live… That’s what co-op is. Co-op isn’t about the characters interacting on the screen; it’s you and I playing on the couch or Xbox Live talking about stuff, engaging in dialogue, and working as a team to figure stuff out.  That’s what real co-op is." - Matt Turner, producer, Army of Two: The 40th Day

“We know a lot of people out there like playing with their friends, with their family, and so we wanted to support that. But the other reason [for the co-op play] is that we kind of wanted to move away from strict survival-horror rules – which are, you’re alone, you have to really watch your back, [and] conserve ammo all the time – and move more in the direction of action-horror.” – John Calhoun, senior designer of Dead Space: Extraction

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