Why OCD seems to focus on one area and not others is not totally understood.
However, it appears that OCD focuses on the things that people are most afraid of. For example, a person with a contamination fear who washes her hands 50 times a day may have no fear of someone breaking into her home, so she doesn’t compulsively check her locks. Yet a person with a fear of burglaries may check his locks 50 times a day and hardly ever wash his hands.
The fear on which a person with OCD is focused will be the one that is most pronounced in that person’s compulsions.
Maybe the person with contamination fears was once very ill and does not want that to happen again, yet her home has never been broken into, whereas the lock checker may never have been seriously ill but has had his home broken into on several occasions. Therefore, their life events have contributed to their fears.
It is also possible that no major events have happened in the lives of these individuals, but they once experienced an intrusive thought about the mere possibility of something bad happening to them in these areas, and that was enough to set off the obsessive thoughts and compulsive behaviors that they experience.