Here's the thing amigo. All the activity of coop and multiplayer is higher in New Game +.
Unfortunately, they actually built the game around New Game +. This includes multiplayer.
Invasion rates are much, much higher in new game +, and it's far easier to get summoned as a red phantom if you use an orb or the red soapstone.
Co-op is a little iffy. It's always like that in dark souls. The first months of the game are heavy on co-op, and as the game ages the player base shifts to the multiplayer since everybody and their grandmother can already solo what the game dishes out.