OCD is often a result of a big heart in an intelligent person who relies on their own understanding.
One of the first things I found when studying OCD is that people who identify or are identified with it are regarded as tender, thoughtful, and intelligent people. Now obviously a big heart and intelligence aren't bad (1 john 4:8, 1 Cor. 1:24). It's the last factor I mentioned above combined with these two that make what the world labels as OCD manifest itself. Because of the big heart a person is often emotionally moved in a big way. Because of intelligence, the same person has an analytical mind and is prone to rely on their own understanding.
Fear and hurt then may be when said person's heart has been hurt by an error or something missed by the brain and all of it's facets. Everyone does this, those with a big heart however are on the extreme end where it's recognizable and therefore labeled OCD. Because they are compelled by a big heart they are equally hurt by any miscalculation of the brain which they were previously relying on. This drives the person to improve their thinking that they rely on (often people with OCD are perfectionist) to prevent such an occurrence in the future.
Therefore, because other than God nobody can have absolute knowledge, and because their reliance on their knowledge, the OCD sufferer gets caught in a loop. Their sensitive heart compels them beyond their contemporaries and therefore their reliance on their brain to make absolute their path is even more driven in that it needs to protect them and therefore rule out all possible negative variables. Because their brain, having been "fooled" in the past (which is inevitable as nobody but God had total knowledge) no longer wants to grant certainly to the sufferer thereby giving the heart an "all ok" or "all clear" to move forward for fear of a variable they missed, can't understand, can't rule out, and therefore can't accomplish absolute, risk-free certainty (which is why those like me with OCD have commitment issues which we'll get into later in this article). The disciples initially struggled with this too which I'll elaborate on later (Luke 24:41).
Because we've forgotten to lock a door in the past we have a hard time assuring ourselves we've locked it this time. Because we can't physically see our hands are clean and free of contaminates we wash them again. Even when we have rational assurance that the door is locked (after all we saw ourselves check it over and over) and even if we could physically see germs and therefore have rational assurance our hands our clean, because we (our hearts) no longer trust our brains fully, we can never get that "all clear" feeling that our heart so desperately wants. Therefore we repeat rituals in a vain attempt to get that all clear feeling to go through. (Keep in mind feelings are a matter of the heart.)
On larger scale items our heart, even more so, looks for "all clear" messages from our mind on things the mind can't fully comprehend (1 Cor. 13:12) and has no authority to send such assurance. Therefore we get hung up on things that seem irrational, even to ourselves, things like "how do I know I won't hurt that person" or as a Christian, "how do I know I'm saved", "how do I know for sure God exists", or "how can I rule out that I haven't committed the unpardonable sin". We look about and see people who have assurance in these things and consider them ignorant "how can they really know". And being tender hearted, we actually don't want to talk with them about this (among other reasons such as fear of them thinking we're crazy, etc.) because we worry we might propel them into the same fiery ordeal.
Because of our heart's need for certainty and the damage done to it in the past, and given the intelligence of the sufferer's mind and its analytical ability (thinking outside the box) we start factoring in outlandish variables (that in some circumstances can be a strength which I'll discuss later regarding obsessive variable thinking). These "outlandish variables" manifest themselves as intrusive thoughts that are usually unwelcomed by the sufferer. However, because of their compulsion to rule out all negative variables their brain retains these thoughts in a futile effort to be done with them. A vicious cycle of trying to fight fire with fire.
OCD operates in our mind much like the HIV (or AIDS virus) operates in the body. When a person gets sick with a normal virus, the virus enters the body and the infection begins to spread. Then after that our body's immune system is alerted and reacts. The immune system sends CD4 cells, also known as T-helper cells to go fight the infection. After some time goes by the infection is overcome by the immune system and we revert back to normal. The HIV infection differs in that, unlike a traditional virus which hits various parts of the body, HIV specifically attacks and infects the T-helper cells that have come to kill it. Other viruses spread from one defenseless cell to another but the HIV virus specifically attacks the very cell that came to kill it. So what does the body do? It sends more. And the more it sends the more food the virus is receiving and able to spread faster. It's like trying to put out a fire by throwing paper on it and then when that doesn't work we try again by throwing more paper on it.
Feels like OCD doesn't it?
It's interesting because the HIV/AIDS virus doesn't actually kill a person. It simply keeps the immune system occupied and subdued so that the host is more susceptible to other threats that wouldn't otherwise be a big deal. A person with AIDS can be done in even by a common cold because there's nothing left to fight it. In the same way a person with OCD suffers from a depletion in mental and emotional resources to deal with things in life that otherwise wouldn't be as big of a deal. Just like what the HIV/AIDS virus does to the immune system, the more an OCD sufferer focuses on the object of obsession the more the powerful the obsessing becomes. The more resources you spend searching for the "all clear" feeling, the more "all clears" the disorder will infect and the end result is more fronts to battle then what was there to begin with.