Be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ's sake hath forgiven you. The last clause is the whole engine. Paul does not ask for kindness as a personality trait or a social lubricant. He asks for it as an echo.
The verses just before are about the ordinary machinery of falling out with people: lying, anger held past sundown, bitter speech, evil speaking — the sort of talk that is technically accurate and designed to damage. Paul names these as things that grieve the holy Spirit of God, which is a striking way to put it. Not that they break a rule, but that they hurt someone.
Tenderhearted is the harder word. Kindness can be performed at a distance; tenderheartedness means letting someone affect you. It is what you lose first when you have been hurt by a person and have decided, sensibly, to armor up around them.
The measure Paul gives is uncomfortable and freeing at once: even as God for Christ's sake hath forgiven you. Not proportionate to what the other person deserves, or to whether they have apologized properly. Proportionate to what you have already received. Which means the question shifts from have they earned this to have I forgotten what I was given.