> no one has ever told me what the causes are.

Many of "them" don't know. They appear not to feel they should know.

My ideas: it is a mental mechanism to "bind" anxiety by enclosing it with invented "regularities." An internal mental/emotional "system" is out of balance and is trying to restore itself and finds obsessive and compelled things the only way to try to do it. Of course those things are only partly successful, because they don't restore the damage that caused the system to get out of balance in the first place. The damage that caused the anxiety in the first place. You have to find out what that was, to root out the root causes.