Feeling annoyed

So, why exactly do certain feelings exist? For example when you're mad or sad or don't want something, why does it exist?
At this point I feel like a hopeless case again: it's not so much that I want to kill myself, but I don't want to be alive all that much either.
I don't like thinking about it. I don't like thinking about the future. In the end, I will lose everyone: I will lose Dennis, I will lose my family, I will lose my friends. I will lose Dennis if we'd break up, and if we won't, I would lose him to death. I will lose my family if we wouldn't have much contact which is most likely, and if we do keep in contact, I will also lose them to death. Same with my friends.
Why do animals die? Why do people die? Why are people sad? Why is it normal nowadays, to pick on someone, to make someone see only one solution: death. Nowadays it seems to be normal to not want to live, or to feel bad.
At this point I feel lonely again. I want to crawl in a corner and cry, waiting till I finally fall asleep, partly hoping it's all over then, while another part of me will still hope I wake up, to see the sun again, to know I'm still alive so that I still have a chance to find a hobby, to meet Dennis, and maybe to get more friends too.
So why is it true? Why do people have a good or positive side, and also a bad or negative side? Why were we created that way?