Why did House ask for Ketamine?

Future Spoilers (Sorry, I can't figure out how to add multiple spoiler covers):

A running theme throughout House is his battle against pain.

He gets to the point where he begins taking experimental medication because he believes it will help heal his leg.

It's fair to say he begins getting desperate for relief.

I also recall a scene where he zones out in front of some generic gossip show, and just closes his eyes. I always thought that he looked tired in that scene, and that he looked like he needed to let his mind rest.

Limiting your brain power is certainly one way of restricting the work your brain has to do.

Who knows exactly what was going through his head. Like I said, he gets desperate.

The whole hallucination could be interpreted as House arguing within himself whether to take ketamine considering the risk of his mind being affected.

The shooter said:

You think that the only truth that matters is the truth that can be measured. Good intentions don’t count. What’s in your heart doesn’t count. Caring doesn’t count. A man’s life can’t be measured by how many tears are shed when he dies. Just because you can’t measure them, just because you don’t want to measure them, doesn’t mean it’s not real. And even if I’m wrong, you’re still miserable. Did you really think that your life’s purpose was to sacrifice yourself and get nothing in return? No. You believe there’s no purpose to anything. Even the lives you save, you dismiss. You turn the one decent thing in your life and you taint it, you strip it of all meaning. You’re miserable for nothing. I don’t know why you’d want to live.

Then House eventual apology to the shooter and his shedding a tear showed the "humility" side in him that made the final choice.

S03E01 explained, when Wilson said:

I heard you were watching surgery with a patient's family, talking to a patient's family. It's because of your hallucination, isn't it? After you were shot. You chose life. You decided you wanted meaning, so you took a case with no mystery, something any doctor could do, a case with no upside except the satisfaction of helping another human being.

and House didn't deny it.

I'm taking a stab at this before I watch season 3, episode 1. Perhaps House realized THROUGH his series of hallucinations that his intelligence would not be compromised, even if given the ketamine. His ability to theorize, his ability to OBSERVE and define truth from falsehood, proved to be true WITHIN his episode-long hallucination, and he witnessed his own ability to define truth from falsehood in his own hallucination (when the bullet rolled out from the patient's hand).

That being said, when he woke up from the attack, he realized he could relieve his leg pain without compromising his intelligence.