My husband Dave is often one of the last people to get mad or frustrated at another person. He will say something like "There's no way to tell what they're going through."
Although he certainly does sometimes get frustrated, it's more common that he finds himself wondering what might actually be happening for that person that he has no way of knowing about and treats them with good will instead. (He usually denies he's as forgiving as I say he is, but the tougher things get, the more understanding he seems to get.) Here's a video that shows a similar viewpoint: