Read Romans Chapter 12.
"Bless those who persecute you; bless and do not curse."
In this life, people will be unkind to you. People have myriad different ways to mistreat you, look down on you, or make you feel bad about yourself. And the natural impulse is to hate them right back, and feel perfectly justified in doing so.
But resenting people, as the saying goes, is like drinking poison and hoping the other person dies. Resentful, angry, bitter, judgmental thoughts poison your mind and heart, and separate you from God.
That's why Jesus said we are to love our enemies and pray for them. Our sinful nature will give us a hundred reasons why we are justified to think badly about someone, but Jesus' instructions to us are clear and unequivocal, and they are for our own good:
“I tell you, love your enemies and pray for those who persecute you, that you may be children of your Father in heaven...If you love those who love you, what reward will you get?...If you greet only your own people, what are you doing more than others?" Matthew 5:44-48
Oh, but the struggle is real. I can get easily get caught up in obsessive thoughts about all the things I would like to tell someone if I weren't such a nice person, in a word to "curse" them.
God's way, the way of "blessing and not persecuting" is just not natural. You have to train your mind to do differently, to go against the flow.
At those moments when I feel negative, judgmental thoughts towards someone creeping into my brain, I silently say a blessing over them. I repeat the priestly blessing from Numbers 6:24 that God gave to Moses:
“The Lord bless you and keep you;
The Lord make His face shine upon you,
And be gracious to you;
The Lord lift up His countenance to you,
And give you peace.” Numbers 6:24
I repeat the blessing as many times as I need to, until my mind and heart are good, until I feel sweet relief from anger, resentment, judgment or bitterness; until I really mean it, and I want God to bless that person. At that moment, God has blessed me too.