"Have the courage to be kind."
My mother always told me when I was a kid "to treat people the way I wanted to be treated." I think this is a really good moral to live your life by and everyone should mold their minds with positivity.
The truth is you never really know what is going on in a persons life, someone could seem really great on the outside but be trashed on the inside. Spreading positivity and kindness is always the way to go. Whether you spread kindness in small ways such as helping someone when they look like they're struggling, holding a door open for someone, or simply just smiling.
You never truly know what other people have going on in their life. Smiling to someone on the street or saying hi to someone that you see all the time but maybe not talk to could make that person's day a hundred times better.
Just because someone has not necessarily been kind to you doesn't mean you should take it out on other people. Practicing spreading positivity not only benefits the people around you but I also think it helps mental health.
It really doesn't take much to be kind to everyone. Even if they aren't necessarily nice towards you, doesn't mean that you shouldn't be nice to them. It goes with the saying "treat people the way you want to be treated." It might change the way the person viewed you at the beginning of your interaction.
Also, some people are just looking to pick fights and get someone made for the thrill of it, so you also won't give anyone the satisfaction. It's a big mother thing to teach lessons about being kind to everyone and these are probably the most important life lessons.