Don't bring the outside inside.
One the hardest concepts for students to grasp as they start their training in a traditional dojo is to maintain a sense of self-restraint. The dojo is not a place for personal feelings, desires or problems. The more I study Aikido, the more I realize that what I am really studying is the art of self-restraint.
This idea of self-restraint is at the core of Japanese culture where conformity, humility and restraint are paramount. Tatemae and honnen are two concepts that embody this idea of restraint. Tatemae 建 前 is the face you show outwardly and honne 本音 is your true face that you hide inside. Tatemae and honne enable an overcrowded nation to function as a single unit, but it is what also allows students to grow and foster in an environment designed for not only their well being, but their growth too.
This idea of self-restraint begins with students and teachers leaving the outside world outside. People's problems or feelings are their own and thus should not be brought into the dojo. Sensei used to say, "Cut off your head and leave it at the door." This enables the practice to be without prejudice, worry, anger, or anything else that causes a distraction. This lack of distraction environment makes the conditions conducive for learning. From this environment springs the beginnings of not only strong Aikidoists, but better human beings.
So when people come to the dojo they should be ready to learn and help others grow and they can only do this by not bringing the outside inside or as Sensei said, "Leave your head at the door." Oh and this is also one of the main concept or highest teachings in budo too - don't let the outside affect the inside.