Because your co-workers derive their self-esteem from their work

The more they get promoted, the more your co-workers think of themselves.

The more they get paid, the more your co-workers think of themselves.

They treat money and position in the monkey hierarchy as proxies for their value in the world.

You wouldn’t care that this is how they measure their worth, if it weren’t also how they measure your worth.

If they get promoted over you, they think they’re more valuable than you.

If they get paid more than you, they think they’re more valuable than you.

They make it clear to you that they’re worth more than you, in ways subtle and not so subtle.

Sometimes they come close to convincing you that you’re worth less than them.