Byron Katie says that "when we believe our stressful thoughts we suffer, and when we don't believe them, we don't suffer."
Stressful thoughts like... 'I'm not good enough, he should treat me better, I can't do that, she is lazy, the weather is too cold, he is weird, she is fat, I have to succeed, she can't be trusted' etc. determine whether we live in peace or stress.
Once believed, they run the show, finding all the proof they need to go 'See, I am right!'.