Humans are social animals and we were (are) unable to survive alone. That's why it mattered to be accepted within a tribe.

What used to happen when a tribe did not accept someone or reject them? They would be unable to survive - i.e. they would die.

Even though it is unlikely that you will die because of this nowadays, feeling underappreciated will cause you stress for these primitive reasons encoded in our DNA.

More importantly, you may start overexerting yourself just to get some appreciation, creating additional stress.

What to do about it?

While sometimes you should change your environment, it's not always possible - e.g. you need that job. You can try to offset it by:

1. Showing appreciation - leading by example

"Be the change you want to see in the world" - even if the person in question doesn't start showing you the appreciation that you deserve, gratitude by itself has been shown to reduce stress, depression, and anxiety.

Moreover, you only need to feel grateful rather than be grateful towards someone - your brain won't know the difference.

To start off, try thinking of three things that you are grateful for when you wake up. It only takes a minute and the long term effects are substantial.

P.S. You don't have to be perfect with this and there's no single correct way to do it. Give it a try.

2. Forgiving

Forgiving is a great tool to free yourself when you feel resentment towards someone. Similarly to showing appreciation, you only have to feel that you are forgiving the other person, you don't have to do it in person.

Truly forgiving can take a while but trying to forgive every day will reduce the stress and resentment for the other person. You might actually see things from their point of view, most of the time people hurt others due to their own insecurities rather than purely to do you harm.