Under what circumstances does OCD occur?

OCD occurs when a sufferer reaches a sufficient level of inappropriate anxiety where the subconscious mind produces 'anxious scenarios.'

These immensely imaginative scenarios manifest themselves as the result of the 'what if?' thinking patterns of anxiety disorders. The OCD sufferer thinks, 'if I don't do X, Y will happen.'

Of course, this thought pattern is completely unfounded, but to the OCD sufferer, it makes complete sense to comply even when the sufferer themselves thinks it's wrong to do so.

Fear is an immensely powerful influence, it's started wars! OCD, driven by fear, can cause the sufferer to carry out rituals which most people would consider completely irrational.