You can get depressed for a lot of reasons like, trauma, any kind of abuse, family/friend/people problems, changes in the brain, family medical history, personality, drug and alcohol use. I'm sure there's other reasons as well.
Depression is a horrible illness that effects someone everyday. But its just not being only sad. Its not just for a couple of days. I hate how people say its just a "chemical imbalance".
It can effect your mood, thoughts, appetite, sleep and relationships. I know this because I have it. Everyday you wake up and its the same as all the others days. There no one there for you, no one that understands, nothing fun etc.. Your always empty, sad, angry, lonely, tired, you hate yourself, you don't want to be around, sometimes you actually have physical pain from it. etc. You don't want to get out of bed because you don't want to go through another day. You might push people away even though you really want them around, you will blame yourself you will want to die, you will feel hopeless, you will feel like it wont get better no matter what you do. You might not display your emotions the way you want. You will feel like everyone is against you or hates you.
There's a lot of other things to it but I don't feel like typing them out.
Depression looks different for everyone, not everyone has these exact thought for feeling but I think I got the basics.