What's the causes of OCD?
There is no definitive cause for a person developing OCD, yet I'll do my best to share a few theories.
Biological causes of OCD has focused on a circuit in the brain which regulates aspects of our behaviour such as aggression, sexuality, and bodily excretions. This circuit relays information from the Orbitofrontal cortex (front part of the brain) to the Striatum, to the Thalamus (Deeper parts of the brain) also including the Caudate nucleus of the basal ganglia. When this circuit is activated. These impulses are brought to your attention and cause you to perform a particular behavior that appropriately addresses the impulse.
An example. After a visit to the bathroom, you begin to wash your hands to remove any harmful germs you may have encountered. Once you perform the 'appropriate' behaviour. In this case, washing your hands. The impulse from this brain circuit diminishes and you stop washing your hands and go about your day. A theory has been suggested that if you have OCD, your brain has difficulty turning off or ignoring this impulse from this circuit. Which could lead to you washing your hands again and again.
Abnormalities, or an imbalance in this neurotrasmitter, could be to blame. Serotonin is the chemical in the brain that sends messages between brain cells and it is thought to be involved in regulating everything from anxiety, to memory, to sleep. Medications such as SSRI's are often used to treat OCD. Although it is not fully known why SSRI's seem to help some with OCD.
Brain imaging studues have been used to show the differences between the brains of people with OCD and those without OCD, but the scientific community is split over whether what researchers have found is a cause for, or a result of, having the disorder.
There are many other factors that have very developed theories on this subject. These are just the two that I recall the most about. I hope this helps some.