Board Thread:Dark Souls II/@comment-92.209.130.19-20141008094629/@comment-5264386-20141008103151

Did you, by any chance, join the covenant of the big stone tablet in Majula? If you have, then you've joined the Company of Champions, which disables co-op play (both as a summon and summoning a friendly player/NPC), but doesn't affect invasions or messages. You'll know if you have, because this symbol will be next to your health and stamina bars: Other than that, though, I haven't heard of something that disables co-op specifically.