Exactly this and I have suspected for years now there is a huge gut connection. I am not saying it is the only cause (or even the sole cause) but I do believe for a lot of people it is exacerbating the issue significantly. The gut also has tons of opioid receptors and there are some scholars who now consider the spinal cord to be a serious part of the brain - and the spinal cord directly branches out in very large nerves all over the gut.

I personally notice a large positive change in my mental health when my gut is functioning properly and I am eating the right fermented foods. I do not take SSRI's and have not been diagnosed with major depression. I also find it hard to believe that it is a coincidence that as a species we have been eating and making fermented food for so long and so suddenly discontinued its use in a widespread way and are now seeing increases in many types of disease. Although lack of exercise may very well also be a huge contributing factor.