From Diablo Wiki
Single Player is the primary focus for Diablo III. Not to encourage people to play alone, but to make sure that playing against the computer is the best possible experience, and that cooperation is a feature that enhances the single player experience, rather than anything else.
- For us, a lot of the core game has actually come out of singleplayer, but what we've done along the way, is we've always had cooperative play enabled. So people play cooperatively all the time. Any time anything does work for cooperative play we fix it right away. So any mechanics that don't work, any kind of balancing or tuning that needs to be done, we try to do it simultaneously for both – but we do focus on single-player more.