There are a number of problem areas connected with obsessive or intrusive thoughts or images. We don't have very much control over the thoughts that pop into our heads but we do have some control over how much we engage with them. People suffering from obsessive thoughts tend to think that they have no control over the thoughts, though this can be learned. The common nature of these problems is that they are circular ways of thinking – each time the thinker goes round the spiral, the available evidence is distorted and there is no possibility of any new evidence. There are four areas with different emotional downsides.
Worrying is a form of super-planning. All possible alternatives must be considered and planned for and this can lead to high anxiety and crippling indecision. If coupled with catastrophic thinking (imagining the worst possible outcomes) then the world becomes a frightening place with a disaster around every corner.
Depressive rumination is a thinking pattern associated with being depressed. The same circular pattern of thinking takes place and every thought is filtered through the blackness and misery of depression so that the worst possible meaning, the bleakest interpretation, is made each time, resulting in even more lowered mood.
Rage. This is the state of mind of obsessional thinking when angry; the sufferer will typically spend time thinking on thoughts of revenge as well as thinking of everything that happened with thoughts of "was it my fault?" and "what could I have done differently?" and ends up making themself even more angry.
The obsessive thoughts experienced by someone with Obsessive Compulsive Disorder are similar to the first three cases; they have triggers such as hygiene or security. The difference is that the sufferer may compensate for them with compulsive rituals rather than to engage with them.