First, if you're having pain from something, you don't anymore, so there is that sensation of relief. Many people also experience a side effect of elation, a sense of well-being, kind of a kick, an energizing feeling, like you're able to do more and just feel better. I think it varies somewhat with the individual.
While that may not sound all that bad or no big deal, it is because, like anything, if you come to depend on it for just normally feeling well, when you don't have it anymore you feel really, really lousy. Not a good condition to be in, I am guessing.
First, if you're having pain from something, you don't anymore, so there is that sensation of relief. Many people also experience a side effect of elation, a sense of well-being, kind of a kick, an energizing feeling, like you're able to do more and just feel better. Also, the longer you've been in chronic pain, and/or the more intense the pain, the bigger the "rush" (no pun intended) when something suddenly makes it go away for a while.
To us a hamburger when we're a big hungry is just a hamburger. To a truly starving man it's heaven.