Why would they be able to make you unhappy but not be able to make you happy? How can they affect one but not both?

Doing a thing „A“ is not the same as not doing a thing „B“ that looks like the opposite of A. E.g.: not healing your broken leg is very different from breaking your leg.

By being assholes. It happens

They can make you unhappy by being not nice with you.

They can help you being more happy by supporting you.

But if you're really unhappy about your life or work, that's usually not enough