It can be a challenging skill to apply, and you need to use your judgement to discern whether the other person is in a place to engage with what you say.
One comment I'd make is the difference between "valid" and "rational". Emotions and feelings are always "valid", in the sense that they are a natural consequence of events and prior conditioning. But feelings are rarely "rational" - they often don't reflect the complete truth of a situation. For example, suppose someone says "Jennifer sent me this short snippy reply today, I swear she's upset with me about something and won't tell me what it is". It is perfectly legitimate to validate that you can see where that fear comes from, but nevertheless offer alternative possibilites: maybe Jennifer is going through a tough time personally, or has a really tight work schedule at the moment. You don't have to fully buy into someone's thoughts and feelings in order to help them process them. In fact this is rarely going to help.
Other people have given good insights, so I'll instead describe one of my pet theories.
Given by how we talk about emotions, I think they are "rational", but operate under a different set of rules than we normally apply to "rational" thinking. In fact, feelings are deeply intertwined with our supposedly "rational" thinking, to the point where I don't think there is a significant boundary. The lack of information is prevalent when feelings are in play, and I believe the same is true in general. Even physics feels far different than pure mathematics, after all. Instead of deferring to conventions in how to act when feelings are involved, as if they belong to a wholly different and mysterious world, we can make sense of the entire world. But of course, empathy, kindness, and good judgement are not exempt. None of this conflicts with what you're saying, but I think a subtle shift in mindset will be fruitful in applying it.
Indeed, the more strong the feeling, the less rational it can become, even though the feeling is there for good reasons. A pure rational solution won't help, pure empathy as well not.
> Emotions and feelings are always "valid", in the sense that they are a natural consequence of events and prior conditioning.
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.