I got this question asked to me privately in a forum.
First, its interesting that its a man asking about men, but women also do enjoy wearing nappies and rubber pants!
Recent studies show it harms nobody, and this has gone now even into Academia (and I can share with you some studies for those who are curious). Only when its obsessive and interferes with your normal daily life an intervention might be needed.
I don´t think there is one only reason for wanting to wear nappies that applies to everybody. For many (especially the youngest ones) is recreational and a trend. A way to lose control, and for that reason it is often associated to D/s activity. One more way to feel humiliated and lose control. For others its been a way to cope with a medical condition. Others feel that being in that mental space is wonderfully relaxing and it helps them to cope with every day's stress.
It seems to be easier for women to find a partner who will care for them, than for a male to find a female partner who will want to do the same, and I am under the impression that there is something associated with male vulnerability and the difficulties of many men to be able to integrate that very human vulnerability into their every day personality, as they feel constricted by societal rules of a still patriarchal society.
And others just need a hug.
And they feel comfy having it that way. :)
For a deeper approach... have a conversation with me. :)