To read the rest of Mark Sichel's post on resentement on his blog, click the tiny link below the paragraph:
Poisoned Mind, Poisoned Body
Take a look again at that quote: "Living with resentment is like taking poison and expecting the other guy to get sick." This makes vivid one of the most crippling aspects of resentment—one you may be experiencing right now. If you're thinking about ways to get even and prove to another person that you're right and they're wrong, you need to remember that the person who is the focus of your animosity may be feeling just fine, enjoying life, and perhaps not at all troubled by any of the interactions that are renting space in your brain. Ultimately, resentment hurts you far more than the person toward whom you bear a grudge.