Doctors are still working on what exactly causes depression, but it's important to think of it as an illness rather than just a mood or phase. Try looking at it this way: people with diabetes have trouble with a certain natural chemical in their bodies, and they need to take insulin shots to stay healthy. If you're depressed, the same sort of thing may be going on. The sadness comes from your body, and when bad or tragic things happen, you could be more likely to get deeply sad or anxious than other people. Or, you might become sad even though no bad things have happened to you recently.
The human body (and that includes the brain) is a very complicated thing. There are lots of different hormones and chemicals sloshing around in there, and each one plays a different role in keeping you happy and healthy. It could be that different levels of these chemicals can lead to some people being more likely to suffer from depression, while others are less likely.
When you're a tween, the process of puberty can also do crazy things to your body's chemicals, especially your hormones. It's normal for this to cause mood swings and other behavior like acting up or feeling extra-sensitive. However, there IS a difference between that and chronic depression. Doctors and other professionals can help someone figure out exactly what's happening.
To put it simply, when it comes to suffering from depression or not, your biology, including your genes, may be as important as the things you have experienced in life. Clinical depression can actually run in the family, like other diseases. So people who have "nothing to be depressed about" can still be depressed. This is all normal and natural, because everybody is unique.
Does this mean that if you experience personal tragedies or if depression runs in your family, you're doomed to be depressed? Does it mean that you shouldn't bother to fight depression because it might happen naturally?
NO WAY! Just like diabetes or other health problems, depression can be treated and brought under control. It's very important to do what it takes to treat depression. Ignoring any health problem is a very dangerous thing to do, and depression is no exception.
So, here's the hard part: Asking For Help.