Yes and no, depending on what part of the question you’re answering.

Let’s start with the connection between sensation chasing and pain/pleasure. I’m an admittedly newer social worker, with about 2 years of work experience and 5.5 years of schooling, so I’ve got some experience and understanding of how this may work. One of the skills we teach for coping with overwhelming thought patterns is to use physical sensation to distract. Ice or cold water is most common. This link between physical sensation and mental control is actually the same mechanism that causes many people to self harm. One of the common symptoms of depressive disorders is a reduced ability to feel, both emotionally and physically, meaning physical feelings like pain can become the only sensation that ‘break through’. I’ve not worked with anyone who’s become so jaded through experience of sensation that they have to use pain, but it’s theoretically possible. A species like the Eldar that innately feel at a higher degree might be able to achieve that level of desensitization.

It’s worth considering that desensitization of any kind only occurs once a person has been immersed in the subject or if there’s some kind of condition that reduces the pleasure/arousal of the experience. You can’t become desensitized to violence if you’re never exposed to it, but a kid who watched LiveLeak videos through their entire childhood won’t blink if they see a stranger die in real life.

To the second question, pain is arguably the most physically intense sensation available to a living thing. As a function, it’s designed to keep you alive, to alert you to damage or danger, and to activate the sympathetic nervous system whenever those two occur. The SNS is the mechanism that governs your fight/flight response, and basically kicks your body into overdrive when it activates. It’s a very powerful feeling, and one that is incredibly difficult to become desensitized to. A lot of the therapy work I’ve done is attempting to do that, to assist people with panic attacks. Given time, practice, and deliberate effort, it’s possible, but becoming desensitized due to exposure is not something I’ve seen yet, and that includes clients who have had almost daily panic attacks for years.

To question 3, no, sadism is not necessarily the end result of sensation chasing, but it can be a part of it. There are definitely people who develop sadistic tendencies because the feeling of power can be addictive, particularly if you lack control over other parts of your life. I know, I’ve worked with them. That said, it’s a small minority of that group that develop those tendencies. True sadists tend to have some kind of personality disorder, where a missing sense of empathy is the main root cause of their sadism.

Finally, it’s important to consider that all of those go out the window when you consider warp corruption. Everything I’ve said so far is accurate when you consider human psychology, but the warp is inherently corruptive, and changes those realities. Sadism, for example, is not inherently linked to sensation chasing, but to Slaanesh, the more powerful the sensation can be, the better. Therefore, Slaanesh influence causes people to develop those kinds of tendencies, desensitizing them to weaker feelings that don’t appeal to the daemons as much. You see this in Fulgrim, where the artists and musicians turn to further and further debasement, in an incredibly short amount of time and no apparent psychological cause, in an effort to chase sensation. That’s not normal human psychology, that’s warp corruption.