I think it's kind of a stereotype that metal is supposed to be negative, sad, angry or violent, and that everyone who listens to it are walking around feeling those things all the time.
That's what I was hit with by my very religious family and school as a teenager. They were terrified of me becoming a satanist etc.
As I've grown up I've learned that metal is very diverse, there's something for practically any mood.
I'd say that some bands do an excellent job of expressing the darker feelings in life, like mental illness, drug use, breaking free from oppression, being unhappy with politics, religion or our society, etc. And that the heaviness of the instruments and slightly melancholic tune of the music can make those messages sound even more powerful. And I find comfort in that kind of music if I'm feeling down myself.
But that's not all bands or all of the time.