Perhaps they have an objective view of the situation and can see that it is not as bad as you are making it out to be. Or, it could be someone being patronizing or condescending - depends on the context. If you are arguing with a spouse and they dismiss you as "overreacting" - that is mean spirited and a way to avoid accountability.
If you see a spider and start to hyperventilate and it's non-poisonous (ask me how I know lol) and someone says "calm down, you're overreacting it's a harmless creature" that is totally different.
People do overreact to things and if we are respectful about how we let them know they are, perhaps it can spare them some anxiety and pain?