At what point does politeness begin to indicate a lack of self-respect? At what point does self-respect become selfishness?

Of course it’s always good to be polite. But if you observed that someone was always apologizing about every little thing, for example, do you think that would be an indication of low self-esteem? Where does politeness cross the line into low self-esteem?

Conversely, self-respect is vital to a person’s happiness. But at what point does refusing to take shit from anybody become selfish? Should friends be willing to tolerate a little shit here and there from each other?