When Hassan was around, ‘the oxygen seeped out of the room.’ What is happening here?

Question

I am looking for an explanation as to what the phrase "oxygen seeped out of the room" means, in The Kite Runner by Khaled Hosseini in the following passage. For example, I can write that it means, "Amir was uncomfortable", but I need a slightly more elaborate explanation.

Hassan milled about the periphery of my life after that. I made sure our paths crossed as little as possible, planned my day that way. Because when he was around, the oxygen seeped out of the room. My chest tightened and I couldn’t draw enough air; I’d stand there, gasping in my own little airless bubble of atmosphere. But even when he wasn’t around, he was. He was there in the hand-washed and ironed clothes on the cane-seat chair, in the warm slippers left outside my door, in the wood already burning in the stove when I came down for breakfast. Everywhere I turned, I saw signs of his loyalty, his goddamn unwavering loyalty.

Answer 1

Wiktionary cites suck the oxygen out of as a variant of suck the air out of. The definition is as follows:

Alternative forms

• suck all the air out of
• suck the oxygen out of

Verb

suck the air out of (third-person singular simple present sucks the air out of, present participle sucking the air out of, simple past and past participle sucked the air out of)

1. Used other than figuratively or idiomatically: see suck,‎ air,‎ out.
2. To dominate or overwhelm, preventing anything or anyone else from receiving attention.
3. To destroy; to cause to become lifeless and empty.

Amir says that Hassan's presence overwhelms him and captures all his attention, He treats the metaphor of sucking oxygen out of the room literally, saying that the guilt he feels toward Hassan makes it hard for him to breathe:

My chest tightened and I couldn't draw enough air; I'd stand there, grasping in my own little airless bubble or atmosphere.

Even when Hassan isn't in the room, the services he does for Amir make Amir aware of his presence. Just as a person in a room low on oxygen would not be able to focus on anything except trying to breathe, Amir can't focus on anything except his feeling of guilt and discomfort centered around Hassan.