[mickronome]

Then, can the lack of serotonin really be the main, or only cause of depression ? If people with sufficient serotonin levels can get depression, then how can we argue that the actual increased seratonin levels is what helps people recover from it ? It seems like a quite strange argument to me.

[vertex-four]

Because depression, like most psychological diagnoses, is defined as a pile of symptoms, not as a specific illness like the flu. We don't know all the different things that can cause depression.

One of the things that we're reasonably sure can cause depression is decreased levels of serotonin. That doesn't mean there's not other things which cause more-or-less the same set of symptoms.

EDIT: On a note related to your girlfriend... people can appear externally happy while being seriously depressed. From talking to me irl, you'd likely never guess that I self-harm, have no motivation to do anything, and wish I were dead. You can't blame someone for wanting an escape from that.

[khed]

Do you have a citation for your claims? Specifically that low serotonin levels CAUSE depression in humans?