What posses a man to cause GBH?

There can be a number of reasons and this isn't going to be exhaustive by any means but the explanations off the top of my head could be along these lines

Someone with a tendency towards violent crimes like that, and related ones, usually has a pretty bad lack of restraint (obvious) but the reason behind it might not be quite so clear. It could be down to a neurobiological explanation, akin to psychopaths where their executive judgement/functioning is impaired so they tend towards violent impulsive actions.

A normal person when angered would think 'God I want to smack him in the face' but someone with an impaired ability to restrain themselves might simply attack the source of their anger and disregard any consequences that might follow because they simply don't factor into their decision making.

The area in question is the prefrontal cortex, which you'd hear a lot about when studying psychopaths, and I reckon that some of that can be applied to people who aren't clinically a psychopath but they have strong violent tendencies.

You can also look at it from a behavioural perspective, where it is a learned response to certain stimuli, a particularly violent upbringing or abusive household can leave a mark to that effect - where excessive violence is 'normal' and so they never learn that actually it's not okay to do that to people. From what I remember, an upbringing like that can lead to developmental problems in the brain particularly to do with the PFC (from the psychological stress, not from being smacked in the head as a child).

It's a bit of an open question though and there are more explanations for a person's violent behaviour than there are stars in the sky, so you tend to have to crack open a particular person's mind and start rooting around to get to the specific causes of an incident rather than a generalisation for GBH.