It is said that most humans have distressing thoughts that can be of a violent and/or sexual nature, but they simply don't seem to make much of an impact. They don't tend to stick in the person's mind, but more importantly, the person can dispel them as being just a thought.

So why does this "normal" thinking process affect people with OCD differently?
It is what you do with the Thoughts/Images which is important and people with OCD who have these types of obsessions can't dispel them as easily. They have a constant feeling of:

"What if I do this?"
"What if these thoughts are for real?"
"What if these thoughts say something about who I am as a person?"

Part of them knows very well this is just a thought, but the OCD part continues with its relentless doubting. So they try to find out the whys of these thoughts or maybe start to hide all the knives in the house.
Soon enough they will realize that reasoning doesn't help and avoiding isn't a real option either. The only relief, although temporary, is found in performing mental and/or physical rituals which will actually reenforce the initial obsession.

Once the obsession occurs, the mind will try and find a solution to these absurd thoughts. The sufferer wants to find an answer as to why they have them and how to prevent having them, thinking that the key is out there somewhere if they just look for it long enough.
But there is no such key. Reasoning with your OCD, finding a logical answer to what is going through your mind is not the solution.