If you must, always show kindness as a personal policy, and not based on your naïveté about human nature.
In other words, don’t be quick to overextend yourself for others, if you don’t want to get hurt. Have signed contracts or clearly spelt duties in your social relationships, and understand that situations of force majeure and other extenuating circumstances are an integral part of social relations.
There’s more than enough evidence to support the claim that humans can be very vicious when it comes to pleasure-seeking and pain-avoidance. And it will serve you no good to find out because you were naive.