So keep in mind im not a psychologist i am just going off of personal experience and why similar characters in other media do similar things.
My theory about why Douma is the way he is:
So right off the bat Douma was raised by cult leaders to be a cult leader and from what i have researched (because im a horror nerd and im curious about such things) that on its own can really fuck someone up mentally. But that much is obvious; What may not be obvious is that im pretty sure that while he was growing up he was emotionally neglected far beyond the whole “our cult has to have everyone happy all the time” thing he grew up with and this can be sort of seen because of how his parents died: his mother murdered his father and then killed herself without first bothering to make sure there was anyone to care for him or even make sure he was out of the house, leaving him orphaned. Pretty sure any parent that does that while their child is still around and not going to have anyone else to care for him is emotionally abusive (and to be honest probably major narcissists because seriously who actually wants to force people into being happy all the time but a narcissist unless they really dont know any better). Speaking of that Im pretty sure the trauma from that happening right there in front of him (whether he actually knows it or not) is what really broke him, when it comes to his thoughts at the time its actually pretty common to think something along those lines when in shock from a traumatic event (like how people who are being held at gunpoint might think “really this had to happen while i was wearing my new guccis” or something). And then there was the pressure to be the new leader of his parents cult and likely having to lead by example during the general age group where emotional development is pretty important since he was a small child when he had to take over like probably not even in double digits yet; the age where most people are learning what emotions are, why they have them when they have them and how to properly react to them and there Douma is having to be happy all the time for these adults that he’s essentially been drop-kicked into leadership of and not getting the chance to learn most of the other emotions. And thats not even including how he was raised to act as a sort of prophet because his eyes were a literal rainbow. Im also worried his parents were either physically abusive towards him or eachother while around him considering his reaction to akaza literally punching his head off and him thinking that he was just being friendly and saying hello (that or its the fact that he most likely has a pain kink), however what he says to shinobu while shes dying suggests that his parents were most likely verbally abusive to each other and then probably lied to him when/if he asked about it. I think the fact that he goes after women most of the time also has something to do with what happened in his childhood (probably his mother murdering his father and then killing herself as she was technically the one who left him orphaned) along with him thinking women are more rich in nutrients for demons due to the fact that they can nurture babies, similar as to why akaza strictly goes after men wether he knows it or not. Maybe this is why he wanted to keep kotoha/mama hashibira because despite her situation she chose to not abandon her son unlike his own mother (and probably the countless others who were in his cult). I also think this may be why he (occasionally) has to literally mess around with his brain to remember things from farther back in the past, since one type of trauma response is blocking out memories of certain situations, similar situations and the times before and/or after the traumatic situation for a certain amount of time. Another trauma response is denial, insisting that nothing is wrong with him emotionally or psychologically until being completely being proven otherwise and breaking the dam.
Long and short of it is: he was raised to at the very least appear happy all the time by emotionally neglectful parents, became orphaned through a traumatizing series of events, got drop-kicked into the most likely very stressful position of cult leader probably way before he was anywhere near ready to actually take that position and had to lead by example the only way he knew how during crucial developmental years leaving no time to learn other emotions let alone anyone to teach him other emotions. Trauma responses are kinda fucking him over along with all that ^ and probably making him a ticking bomb. Basically it’s not so much that he literally can’t feel human/demon emotion but more so he never learned how to and has worn his “happy mask” for so long he forgot it was ever a mask to begin with and eventually forgot it was even there.
