Whats wrong with reassurance? It helps me

It furthers the connection between bad intrusive thoughts and relief. Therefore prompting more intrusive thoughts to come in the long run. It's like feeding a cigarette craving with nicotine. It's conditioning.

Reassurance will only make the ocd stronger. It may feel good initially, but in the end it will make the ocd stronger.

So reassurance is a compulsion. So yes, it does help relieve your anxiety. The problem is the relief is extremely short lived. ERP will give you long lasting anxiety relief. It also strengthens the OCD thought. You are telling your brain that the intrusive thought IS a threat. So your brain will give you even more intrusive thoughts. It also makes your OCD worse. It also keeps you stuck in a vicious circle. More intrusive thoughts leads to more anxiety which leads to more compulsions.

It will give you short term relief. Once you feel okay with the thought the ocd will build on that thought to the point that you can’t seek reassurance anymore. Which will then increase your anxiety.

Also reassurance stops working very soon and you always have to look for more and new reassurance to get relief, because the ocd always finds more "evidence" to worry you. This leads to more and more reassurance-seeking and in that process you also get triggered more and more, especially if you are looking for reassurance online by researching and googleing. It is a spiral that only leads to more and more uncertainty and fear.