You ascribe intent.
Another driver cut you off. Your friend never texted you back. Your co-worker went to lunch without you. Everyone can find a reason to be offended on a steady basis. So what caused you to be offended? You assigned bad intent to these otherwise innocuous actions. You took it as a personal affront, a slap in the face.
Happy people do not do this. They don’t take things personally. They don’t ascribe intent to the unintentional actions of others.