Take it one step further. Why are emotions viewed in a negative or positive way, as well as why do you feel this way?
Because of the consequences of those feelings that hurt ourselves and other people. Like repressed anger has been linked to cancer. Anger often inspires people to violence or other forms of abuse, while on the other hand anger can also tell us "something is wrong, get away from this person, protect yourself."
Same with depression. Depression often takes away the will to live and interrupts productivity and enjoyment of life...but again, can signal to us that we need to change something in our lives to feel happier and become more productive.
All emotions are useful, including painful emotions, or those which frequently are linked to destructive behavior. Anger can help motivate us, sadness can point us in a direction that is better for us. I embrace all feelings as part of being human.
I, of all people, am all about embracing every single emotion, and I don't like emotional repression or people who say expressing "negative" feelings at all is impolite or wrong in some way.
So we're very much in agreement on that.
But we label those feelings as negative because of the behaviors and consequences which often follow those feelings in people who don't stop to consider other things.
Like, "I cut my ex-wives brakes because I hate her because she hurt me and she made me so mad. Ooops! I didn't mean to accidentally kill my three children, too."
That's why they're labelled as "negative" emotions - because they cause us to feel pain and are linked to abhorrent behavior.
As I say, though, with the proper perspective, anger and sadness can motivate us to make positive changes in our lives.
It's a matter of self-awareness, education, and maturity, usually.