Mortal sin coarsens our consciences, and makes us more likely to do more mortal sins, thus dulling our will and our conscience even more. It gets to be a vicious cycle, and is difficult to break the habit of sinning. And because most (if not all) mortal sins are a matter of choosing self before all else, exerting self-control of any sort becomes harder and harder to do.
It gets so enmeshing that I doubt many in the state of mortal sin would even stop to consider that they are deliberately choosing hell over God, or they may just scoff at the thought if it does occur to them.
In the worst case scenarios, people who habitually sin mortally distort their views on their place in the world so badly that they become unfit to function in human society. They simply do what they want, when they want, however they want, with no view to the rights or needs of others. Psychologists call them sociopaths. If they’re not fit to be around other people, how much less are they fit to be around God, especially if they can’t even see that they have been sinful?
