We're taught to turn the other cheek -- that being kind in the face of hostility is the better way to respond to conflict so love can overcome hate. According to psychologist Clifford N. Lazarus, writing for Psychology Today, that sort of reaction just teaches abusive people that their behaviour is effective. Here's why.
It all comes down to something called the "Law of Effect", which refers to the way people interpret and understand the behaviour of others. For example, if someone treats you poorly and you treat them kindly, the effect produced by their bad behaviour is your affection. By being nice to mean people, you're essentially creating a reward system for bad behaviour.
This doesn't mean you should devolve into a complete arsehole anytime you encounter one, but it is important to remember that there is such thing as being too nice. When someone does something that bothers you, it's important to take that immediate opportunity to tell them. Being kind is often just an excuse to avoid necessary conflict. You don't have to be a jerk, but you do have to confront the situation or risk encouraging the bad behaviour you're seeking to prevent.
Are You Teaching People to Treat You Badly? [Psychology Today]