Twilight
A depressed girl moves to a depressing town and meets a depressed vampire.

New Moon
The depressed vampire leaves the depressed girl.
The wolf boy falls in love with the depressed girl.
The depressed girl gets even more depressed and throws herself of a cliff.

Eclipse
The depressed vampire and the depressed girl are back together and now they are not so depressed.
But the girl fell in love with wolf boy which made the vampire depressed again.
And the vampire being depressed made the girl depressed which made the wolf depressed.
So now they are all fighting and are not happy which is the same thing as depressed.

Breaking Dawn
Bella and Edward are NOT depressed because they got married.
Jacob IS depressed because Bella and Edward got married.
Stuff happens, Bella gets preggo, and dies.
Jake imprints on vamp baby and every one lives happily ever after. Although, after reading this I am quite depressed...