Why do human beings feel the constant need to be validated for their actions/opinions/interests/etc.?

Because for the longest time, we lived in tribes. If you got thrown out of your tribe, that was essentially a death sentence.

You did very well in asking this. You are very clever.

We're a stongly social species

A desire for approval drives social creatures towards pro-social behaviour, which can positively influence the sucessfulness of a species

Everyone wants to belong

Everyone wants allies, close friends, brothers and sisters they can rely on and love and support

In the modern world we have pale imitations and crap. There is no village, there is only a dim landscape through which we shuffle, largely alone.

And so there is craving for some substitute, because the under structure is unfulfilled

What is the point of being self-aware if the only thing you are aware of is your inadequacies and failures? We need validation in order to keep going.

Through the ages, humanity evolved to recognize that cooperation generally leads to success. If others like us, we have more of a chance to survive and procreate. So our brains release chemicals that make us feel good when we do things that lead us to success. It used to be that sugar was pretty tough to get, so our brains evolved to release those nice chemicals when we eat it. So too is it that when we receive validation, the brain releases those chemicals then as well.

Not everyone is consciously aware of this happening. They just know that when they do some things, act a certain way with a certain group of people, eat certain foods, etc. they feel good. They don't know that they may be painting themselves into a corner, so to speak, where the only way they can feel good is by doing the things that give them the most of those chemicals. That's why drugs can be so incredibly dangerous, but it's also why you see people doing things that don't make a lot of sense outside of their particular clique. We bounce back and forth between the things we enjoy and needing that enjoyment of those things to be validated.

Because we’re a social species that often checks ourselves off of those around us. Sometimes it’s unhealthy, but often it’s an attempt to maintain tribe status and to safeguard against misperception.

Your question is wrong. Few if any people need constant validation. The question is what frequency people desire it, and how, and that varies greatly by person.

Insecurity

Skill issue tbh.

Some good answers, but I also want to point out that this isn't actually a universal need.

I think that our social nature, and the way most societies and communities work (particularly in countries that subscribe to exceptionalism) makes them prone to developing these feelings, but they very much are not universal and can even be unleaned.

People generally need some validation but I think you're making an inaccurate generalization

Validation for beliefs like all human beings are the same by asking a question where that is included as some kind of truth? That kind of validation? Is this question sarcastic?