Many spiritual paths believe in possession, and it can be a desired or undesired thing, but generally spiritualists perceive that it always comes from a malevolent source.
Some even believe that when you delve into the astral during such practices as scrying, divination, Ouija board, OBE, etc...you become open to such influences. Some even believe that visiting cemeteries, meditation and dreaming open you up to possession as the disembodied wonder those regions.
Some Nightkind claim that they are possessed by disembodied vampiric spirits or entities, and this is how they obtain their 'wisdom', understandings and special needs. Some feel they have unlocked some door and invited in these entities (human, vampiric, animal or otherwise), others feel that they were taken somehow as in the case of walk-ins or the younger crawl-ins. The truth is people change slightly everyday in accordance to life changes personal discoveries and environmental pressures. Cathartic events, of course, change people more suddenly. Possession/astral influence is often associated with drastic life changing events, like NDE or surgeries.
There is also a fine line between communicating, channeling and possession. Channeling and communicating with spiritual entities can open you up to unwanted energies, and sometimes we simply can't help being affected by astral cooties or yucky energy. It weighs on our shoulders, it drags us down even makes us ill. And often we wonder where its coming from.
Science explains this aberration as one of many forms of psychologic or neurologic mental disorders like dementia, delusions, or schizophrenia. But perhaps there is more to that.
At the risk of being classified
When I enter the astral in deliberate meditation or in ritual, I adopt a different sense of self. Sometimes I see, hear and interact with others while immersed in this space. Some times I loose notion of time and space. Some times I listen, sometimes I am compelled to take heed. Sometimes I feel watched, judged or protected. Sometimes I am drawn to write or create some amazing things.