Many people who believe they are just being struck randomly by depression are simply not aware of the reasons they are feeling depressed. That doesn't mean there aren't emotional/psychological reasons for what they're experiencing---only that they don't see those things as the cause.

For the first decade or so after being diagnosed with depression I believed it could only be explained by brain chemistry or something like that. Only later in life when I learned more about healthy relationships and what does and doesn't work for me emotionally did I realize that my depression was an obvious reaction to the dysfunctional environment I was raised in.

My ignorance of the non-chemical causes of my depression made me feel I was "just struck by it". But there really was a cause in my environment. I believe this is true for many other people as well.

Depression and anxiety in particular I think are better explained as a mismatch between human adaptation and current human environment. They are signals that things are unsafe/insecure. It seems to me more wise to focus on improving our environment than on constantly blaming the brain, whether through the serotonin hypothesis or this new neuroconnectivity approach.