Holly and Auld_Kelpie,
I think most of the time you can have compassion on people after hearing their life story. There was once this really EVIL choir director that was directing a special choir performance that my church was doing. She was a royal pain in everyone's butt! There were little old women that were NOT doing what she told them too ON PURPOSE because she was annoying them. Then the choir director started telling her life story about how her husband had died when she was 35 leaving her all by herself with a bunch of little kids...and on went the saga. So I, for one, felt a little bad for her and decided to give her a break.
BUT sometimes after hearing someone's life story you can LOSE compassion for them. Our family had an aquaintence that was about 45, single, with no family. So we invited her to stay with us for the holidays. She stayed for Thanksgiving AND for Christmas. Every time my mom was out of the room she was flirting with my dad...my dad's one of those guys that think women are "just being friendly" when they're totally coming onto him. I was annoyed that we had to have her there for both holidays, but my mom said, "Honey, she doesn't have any family." When she finally got around to telling us her life story we figured out that she didn't "have any family" because when she was married she got bored of marriage...and her little toddler son was too much of a "responsibilty"..so she left and never came back. She admitted that the only reason she tried to keep in touch with her son NOW is because all the "hard stuff" was taken care of by her ex-husband. We didn't have to have her back for anymore holidays.
This is way better than the old "walk a mile in the person's shoes" saying. "Before you criticize someone, walk a mile in their shoes...that way when you criticize them anyway you're a mile away and wearing their shoes."