From time to time I hear a particular quote attributed to the Buddha. The wording differs slightly. The underlying sentiment is the same: whatever you ponder frequently will become the inclination of your mind. I have yet to find the exact working in Buddhist texts but it seems to be a paraphrase of a quote from a Pali text.
“Whatever a monk keeps pursuing with his thinking & pondering, that becomes the inclination of his awareness.” ~ Dvedhavitakka Sutta, trans. by Thanissaro Bhikku
Basically, if you think negative thoughts, your mind will tend to be focused on negative things. If you think that people are good, your mind tends to believe this. If you view the world, others, events, yourself a certain way again and again, you will become stuck in that world of thinking. It will become your default mode of thinking and being in the world. So what default mode do you want to be in?