The Trans-specie theory
This is the most common theory where the person believes he was born into the wrong body. People do feel as if they were born into the body of the wrong gender, so why not the wrong species? It could be psychological, but for many people, it just seems to happen as some display odd behavioral traits that (after looking into their backgrounds) they could not have learned from an outside influence.
The Genetic theory
Some therians or otherkins claim that their DNA is not human and for various reasons (alien spaceships, time travel, ...) they have a human body and stay on Earth. Unfortunately, no DNA test so far has ever revealed such creature. Another theory states that therianthropy is passed from generation to generation, like body type or mental disorders.
The possession theory
In earlier times the state of the patient was commonly explained as due to possession. Marcellus of Sida reported that in Greece the patients frequented the tombs at night, and that they were recognizable by their yellow complexion, hollow eyes and dry tongue. The Garrows of India are said to tear their hair when they are seized with the complaint, which is put down to the use of a drug applied to the forehead; this recalls the stories of the witch's salve in Europe. In Abyssinia the patient is usually a woman; two forms are distinguished, caused by the hyena and the leopard respectively. A kind of trance ushers in the fit; the fingers are clenched, the eyes glazed and the nostrils distended; the patient, when she comes to herself, laughs hideously and runs on all fours. The exorcist is a blacksmith; as a rule, he applies onion or garlic to her nose and proceeds to question the evil spirit.
The Dual Souls/Walk-In theory
Two souls occupy the therian’s body, one human and one animal, or his soul is part animal and part human.
The transmigration theory
Transmigration is related to reincarnation. It means that a soul who was previously an animal has reborn into the body of a therian, carrying with it some of the memories, instincts, energy and personality of original species. Shifts are then considered as past life memories. Conversely, if a human were to die and be reborn as a pig, this would still be an example of transmigration To a larger extent, according to the Bouddhist doctrine, any soul is bound to reborn into the body of various different species or a higher/lesser entities (god, ...) until he reaches enlightenment.
The dissociation theory
Therianthropy is related to or due to a dissociative disorder, whether it be full dissociative identity disorder or a lesser form of dissociation. Shifting is dissociation from higher thought processes and emotion, the “human side”. The wereside is a mental fabrication, where the person desires so much to be something other that he tricks his mind into experiencing shifts of perception, phantom limbs, etc. More about clinical lycanthropy.
The Everyone’s Therian theory
Everyone’s a therianthrope, everyone has an animal side. But very few people become aware of or accepting of this fact, and so there aren’t a lot of awakened therians. Anyone can awaken, though, and become therian.
The soul splitting theory
According to that strange theory, souls can split into more than one piece, and often do. Soul-pieces sometimes/often fuse onto other soul-pieces, and sometimes a nonhuman soul-piece fuses to a human soul, resulting in a therianthrope.
The imprint theory
Imprinting is a term which is used to refer to how a newborn animal recognizes a parent. For example, a newly hatched baby chicken will imprint on any animal which is present for a length of time after it hatches, and it will consider that animal to be its parent, whether it is a chicken or not. Chickens which imprint on humans will follow them around, and to some degree attempt to imitate them. The more complex the brain of the young animal, and the more social it is, the more imitative its behavior will be. So, if a human baby were to imprint on an animal of a different species, it would consider itself to be a member of that species, and would imitate the behavior of that species. This has been proven with the case of feral children but does not seem to be of great help to explain therianthropy
The Gaia theory
Therians are links to nature and the primal side of humanity, an attempt by nature or deities or some other power to return balance to the world. Therians are to fight for this and protect the environment.
The totemic theory
In some cultures it is believed that everyone is chosen by a totem spirit, often before birth--this totem spirit watches over them, and guides them through their life. Totem animal spirits may confer their own characteristics onto those whom they adopt, and this is another possible explanation for therianthropy. The idea would be that therianthropes are simply more in touch with their totemic nature, or that the totem has had more influence on them.
The abnormal theory
Much of human behavior and behavior characteristics is a genetic trait. So, it's possible if the genetic structure and/or physical structure of the brain were abnormal, this could result (and has been seen to) result in abnormal behavior. If these changes were of the right variety, this could produce "natural" animal-like behavior traits. So, therianthropy might turn out to be a genetic mutation.
The racial memory theory
According to this theory, man would carry genes that not only come from our own hominid species, but a whole variety of species going back to the very first single celled organism from which we originally evolved. The original ancient genes from several ancestry animal are merged with our DNA and show up in the form of memory, behavior, or a physical throw-back.
This explains why there are so many different kind of therians, why there are polytherians (interbreeding of divergent species), and why there are even therians of extinct creatures.
It has also been proven that mammals have REM sleep and therefore dream, which suggest we may all have some kind of common dream genetics inside us.