1. Hurt people hurt people.
2. If Satan can't get me to be despondent through recall of my past sins, he will try to get me bitter and graceless by reminding me of others' sins against me. I may not always agree with his condemnation of myself, but he knows it's really easy to get me to say "Yeah!" to condemnation of others.
3. The key not just to appreciation of what I've got but to thankfulness in all things is recognizing I don't deserve anything good.