If “validating” someone’s emotions comes down to simply saying that, yes, I agree you felt that way, then I suppose that’s true.
But when people talk about validating other people’s emotions it implies that they’re saying the emotional response was valid for the circumstances.
I have someone in my extended family who has a strong tendency to catastrophize and assume the worst. When she was in a relationship with someone who constantly validated her emotions and reactions it was disastrous. It took someone more level headed to start telling her when her reactions were not valid to certain situations to begin stabilizing the behavior.
There’s a hand wavey, feel good idea where we’re supposed to believe everyone’s lived experience and emotions are valid, but some people have problems with incorrect emotional reactions. Validating these can become reinforcing for that behavior.
I’m not saying we should start doubting every emotional reaction or white knighting everything, but it’s unhealthy to take a stance that validating other people’s emotions is de facto good.