OCD is caused by a lack of seratonin not allowing neurons to send information. This is why you wash your hands for 20 minutes when 20 seconds is enough; your hands are squeeking from how clean they are as you wash them under the sink, with red skin, yet you "feel" as if they aren't clean yet, when they're probably the cleanest part of your body at that moment.

Your brain is literally not recieving the message, so you repeat over and over until the message is finally transferred and recieved. This is also why completing rituals is so relieving. You are releasing dopamine, training your mind to continue to perform the rituals. It is a cycle of degeneration. You need an SSRI to remove OCD. Your brain is in a loop of little seratonin -> dopamine relief -> less seratonin next time, that felt nice, we are doing something good!

The preference for dopamine contributes to the delusions, rationizing ridiculous behaviors, paranoia over contamination and so on.