I can't take people seriously when they claim to be depressed
Why can't you take depressed people seriously?
They are just unhappy but claim to have a disorder
People who feel depressed deserve to be treated with respect. It may seem frivolous on the surface, but depression is a proven medical disorder and needs to be taken seriously.
I treat everyone with respect. You are probably just saying that because you are trying to be a good person.
I'm glad that you respect them, but you should also be taking their conditions seriously.  Depression may appear to be just a case of sadness on the surface, but it is sadness that people cannot escape, and they cannot understand why they feel that way.  If they admit to you that they are depressed, it is because it is preventing them from functioning, and they do not deserve the stigma that they receive.