What is the reason that many of the conversations between Kitaru and Tanimura take place through a crack in the bathroom door?

Tanimura spends a great deal of time at Kitaru's house and Kitaru has a ritual that when he returns home, he immediately jumps into a hot bath. In order to talk to Kitaru, Tanimura then cracks the sliding door of the bathroom about an inch so that the two can talk. Tanimura also mentions that if he does not take this action, he must listen to Kitaru's mother drone on and on about her son's lack of study skills and other topics of little interest to Tanimura.