That's a huge question Katrina. I would say that not only do the causes of enema lovers vary, but the anwser may depend on what you beleive on a deeper level. For instance:
Clearly, many people on this forum have this addiction as a result of childhood trauma or experiences which range from nuturing to sexual abuse. On the other hand, there are also many humans that are anally erotic, and get great sexual pleasure from all forms of anal masturbation. Enemas are a natural conclusion for many of them because it is arousing in itself and also cleans them out for further anal adventures. Neither of the two scenarios mentioned are mentally unsound individuals unless they take their passion to an unsafe, unhealthy, or self-destructive level like the Illionis enema bandit.
Likewise, there are humans that are born thrill seekers and boundry pushers, and they too find stimulation in enema play through the competive factor (trying to up your best volume or retention time) and the struggle or the whole scenario. Similarly, there are those that feed off the humilation and pain that can be associated with a punishment enema scenario, and they too find enemas a hot-button issue.
In these modern times, we also have the granola, yoga, pilates, and crystals new-age craze, and those people love them some enemas and colonics with all kinds of solutions and variations. Are they really that different from us in their enema obsession?
Finally, I would argue that enema play is part of human DNA. This is where your beliefs come into play. In basic psych, there are theories that people can have fears about animals and other living beings because their DNA and cellular memory holds instinct from a time when we were the prey and not the predator.
Moreover, from that same evolutionary period following the rise of man, we see the phenomenon of power exchange. Tribal rulers rise from the crowd and leaders are picked. Humans have this in the their cellular memory too- picking someone to give power and control to in exchange for security and comfort. Be it the church, a king, or a president, humans seem quite content to give away control to a higher power in return for answers and safety. Sexual power exchange is a microchosam for this instinct, as some of us play out our evolutionary role as slaves while others become masters.
Enemas too have a great history in mankind dating back to prehistoric times. Every period in history has had its moment of enema madness where constant cleansing is the answer to mans ills. This was true in Latin America, Renaissance Europe, Victorian times, Pioneer America, the Roaring 20s, and every other time. Humans have this practice in their instinct from millenia of training. Even animals exhibit this kind of behavior be it enemas, masturbation, or other alternative sexual play.
It is only a problem if you let the demons out of control when you let them out to play.
Told you it wasn't an easy answer.