The more you seek reassurance, the more you will want it. Reassurance is, in many ways, like a drug.
People who take drugs need more and more of that drug over time to get the same high. In a similar vein, people who seek reassurance want more and more reassurance as time goes on. What you gave them in the form of reassurance yesterday will not be enough today.
Also, people use reassurance to help them put off an unwanted consequence. A drug user who is going through withdrawal wants the drug to stop the withdrawal symptoms, just like someone with OCD wants reassurance that everything will be OK so that he or she can break free of the discomfort of the obsession.
The problem is that if you give the drug user the drug to stop the withdrawal, you are effectively maintaining the addiction. The same can be said for OCD. If you receive reassurance to alleviate the discomfort caused by the obsession, then you never learn that you can handle the obsession without getting the reassurance, and the OCD is maintained and even strengthened.
As with the issue of avoidance, exposure and response prevention (ERP) is a therapy that can help to eliminate reassurance-seeking. A therapist has people with OCD confront what they fear without giving them a great deal of reassurance. As time goes on, the giving of reassurance is eliminated, and the people in therapy learn that they can handle their fears on their own without needing assistance from others.