Playing with friends makes any game more fun though.
In some way in d2 I feel like playing with friends is actually better than in d3.. The ai in d2 doesn't suddenly walk away & rushes your teammate. And because you don't have to kite you can actually combine spells & work together to be more than "2 solo players in 1 game"..
Given the other side is also true: the game is so easy moving around between monsters is the major slowing thing, so the 2nd player is completely unnecessary.
I just wish the AI in d3 was "logical": if I'm as monk on front they should attack me and not that wizard standing far behind me. Nor should they swap targets halfway.