Garbage in, garbage out. What does that mean? Well, if you fill your physical body with junk food, it will become overweight and not very healthy. In the same way, if you fill your mind with negative, unkind, impure thoughts, it will show in your attitude and the way you treat other people.
Fill your mind with true, noble, right, pure, lovely and admirable things and your attitude will be more positive, your energy levels will be higher, you will be more hopeful about life itself.
Now the question is ..Where do you find these good things?
To my understanding, you will find these good things in scriptures/books that teach you love, compassion , kindness,empathy and Kindheartedness
Read it, Practice it , absorb it and implant it in your mind.