There are two pseudo-reasons. One is that long term Vicodin use actually can make a person think that they are in alot more pain than they actually are. It's a side effect of abuse of the drug. This has been mentioned on the show a few times, IIRC. Two is that the Psych med House is on is an SSRI. They are also indicated for treatment of cronic pain. Or, so Wikipedia claims ;)

I have no clue if those factors are enough to have such a large effect, but, there are things to point to.