There are not many things, which – considering his position in life – is rather fortunate. Severus dissociates in two ways. The first and easiest one to discuss is deliberate – while the other is intimately tied to his thousand-yard stare.
Deliberate disassociation generally comes into play when Severus is in an abundance of physical pain. He will retreat behind his occlusion walls and forcibly disconnect himself from what is happening. This is extremely dangerous and unhealthy, and has contributed to both his high pain tolerance and his extensive nerve damage over the years.
Severus will not allow himself to dissociate during Death Eater meetings in the Second War, but he did disconnect during the first, particularly when attacks involved women or children. It was a hard line for him that put him in the line of fire multiple times before he established enough of a position and reputation for himself to not be involved in such events.
Which leads us to his thousand yard stare, which we actually do see in Deathly Hallows with Charity Burbage. Severus was not unaffected by her predicament, but he could not afford to appear anything less than unmoved. He was low key disconnected at the time – focusing inward so that he could present an uninterested visage when really, everything about this situation stands starkly against his morals. Which, while gray in many regards, are starkly black and white when it comes to violence against women.
Severus pulls memories like these into a pensieve as soon as he can in order to disconnect himself from the volatile emotions they build up inside him, affording him the ability to function and – if need be – go over the memory and relay information regarding it without being affected. This is, very literally, due to self medicating himself into oblivion in order to cope, which is, in its own way, a sort of deliberate disassociation.
What can cause his soul to straight up ascend without his deliberate action is forceful, physical action while the other party is inebriated and he is in a position where he cannot fight back. This is very specific and has only happened twice since his father died, but it is notable all the same. Severus just shuts down and responds on autopilot because he cannot functionally cope in that situation if violence in turn is not a viable option.

