There are two different types of depression, the most common being the depression we get when something in our life is wrong or the world around us sucks, this is a natural defense mechanism & not something that needs to be treated with drugs most people will experience this many times in their life & many will seek un-needed medical help for it.

The second is a chemical imbalance in the brain causing you to be depressed all the time even when your life is perfectly fine.

As for why are so many diagnosed with depression well you can thank major pharmaceutical companies for this & shitty doctors, who would rather spend 10 minutes with you and then throw a box of addictive, dangerous, brain cell killing pills at you than get to the true reason why you are depressed.