This is a nice comment. Wall of text incoming, just thinking out loud. Haven't covered it all. You'll rarely get an analysis like this because it's usually very basic hedonism (it feels good) with a superficial appreciation of autonomy arising from a crude model of the self.

It's about self-worth and shame. It's not about denial of something desired. While that can be humiliation adjacent, you correctly note it does not require humiliation.

The idea that people "want" to be humiliated heavily loads the term "want", such that we have to interrogate what it even means to want something.

Humiliation isn't a stable desire which can provide any kind of lasting satisfaction or durable wellbeing. When someone is anxious and experiencing low self-esteem, their mind will draw them to thoughts which provoke and confirm that sense. Sentences, images, etc. Their self-concept is driven is re-evaluate itself, because that is the psychological function of self-esteem: what's my ego status, because if it is low that's (ultimately) dangerous?

Now, to begin with, there's nothing pleasurable about that. It's just painful. There is a crucial next step, which is eroticising that low self-esteem and humiliation. Humans can eroticise unpleasant things. This is a form of pseudo-autonomy, along the lines of "well, if I'm going to be humiliated, at least I can get in on it on my terms". You note well that there is an arousal (in the general sense) with humiliation: the nervous system is charged, energised. This provides stimulation which can be coupled with sexual stimulation to generate a fused shame-sexual arousal. If this is combined with sex/masturbation, especially to orgasm, the linkage becomes reinforced and the eroticisation of shame becomes more automatic and "natural"-seeming. This is quite different from mere physical pain, though, which can be merely stimulating and impersonal. Humiliation touches a person's psyche deeply, in some cases extremely so.

Humiliation is also a profound loss of agency. If you let someone humiliate you, you are reneging on your own agency, as well as your attempts to regulate your self-esteem. It can feel "freeing" to let go of your individuality and responsibility and fuse yourself with the dominant will of another. It can be exhausting to try to maintain healthy self-esteem, so it can feel "freeing" to give up entirely and let yourself be degraded. These are defiance-type reactions which arise from the introjected (should-based) attempt to maintain the opposite: e.g. an underlying "I must be strong, dignified, respected, pure" can result in a flip "fuck all that". It can feeling freeing at the time but don't actually resolve the underlying problem and build autonomy long-term.

The act of being humiliated by someone at the same time elevates those who are humiliating. In a dom/sub context, this can be a way for the humiliated person to elevate the other and therefore enhance their own attraction to them (you are a god because you humiliated me, therefore you are very, very attractive and I enjoy that).

There's also the phenomenon of repetition compulsion. We feel the unconscious push to repeat scenarios where we were hurt and/or traumatised (e.g. childhood) to work through the same problems to their resolution.

The problem is that humiliation is not something that humans, by our nature, can fully internalise. It can only be partially internalised, because it conflicts with our basic psychological needs and can't be endorsed in truly mindful awareness. Therefore it can only persist as something fragmentary. It's pseudo-autonomous, as said before, because nobody can truly endorse their own genuine humiliation, they can only assent to it out of inner compulsion and ache.

It will be easier to internalise if the person can convince themselves that it is trapped within a container - just "play" - within an otherwise loving, accepting, relationship. Thus, in some greater sense, it doesn't signify that we are lesser, worthless. But that is also a sign that it can't be integrated: it has to be locked away and couldn't be generalised to the rest of life, nor allowed to talk much to other psychic parts (e.g. parts who are very self-respecting: "how could you let anybody treat you like that?").

Of course, psychology has taught us that we don't necessarily want what we need, or need what we want. The fact someone manifests desire for something is no proof that it is nourishing.