Squeezing the brain how to say "Good Bye"

Japanese people are intelligent and strongly inhibited. They cannot be rude even if the want to. The language has no verb equivalent to "must" to express obligation, so the poor Japanese must torture his brain to say, for example, "Bye, I have to go". The newspaper Japan Times explains:

"Conditional forms are commonly used to express obligation, as if to make up for the nonexistence of a verb for “must.” In fact, the Japanese way of saying “something has to be done” is more like “it’s no good if it isn’t done.” If I have to leave now, for instance, I announce this by saying, 行かなければならない (ikanakereba naranai), or shorter, 行かなきゃ (ikanakya), which is literally, “It’s not OK if I don’t go.” Using two negations, I leave through the back door, as it were.

Even the well known farewell formula さようなら (sayōnara) is derived from the if-construction 左様なら(ば) (sayō nara ba). It’s meaning? Well, so long, “if that is how it is.”

 You have to live long time in Japan to get the meaning of their tortuous circumlocutions.