Byron Katie says that "when we believe our stressful thoughts we suffer, and when we don't believe them, we dont 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, they shouldn't ignore me, my Mum is mean, war is painful, I have to be nice, my boss is a bully, her laugh is too loud, I hate my body, she is sad, cancer is horrible, I need a man, 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!'.