They have known each other since their early days as Seiyuu, on top of that they share same hobbies in Gaming (most of the game they played are same with each other). And Nakamura is one of those people who actually always get the references/jokes from various anime/games made by Sugita (for those who don't know, Sugita like doing this almost every time).
If you are curious, you can try watching "Tokyo Encounter". It's a game show hosted by those two, where they play various games (and sometimes random guests like Takahiro Sakurai, Kaji Yuuki, etc).