I think a lot of the depression with kids is anxiety-driven. Nowadays, kids can watch people getting murdered on TV every day. Luthar's Connecticut study shows suburban teens are more anxious than inner-city kids. Greater isolation also may be spurring childhood depression. Kids used to grow up in large, close-knit, extended families.
Children are alone more now.
Individualism is rampant, and it's not a good buffer against the defeats we all face. Seriously depressed teens are seen by themselves and teachers as less socially adept than peers. Kids who believe bad experiences are the result of permanent causes in themselves, not changeable behaviors or conditions, are more apt to be depressed. There's a strong stress-depression link in teens. Depression doesn't hit the two sexes equally after early adolescence.