In this article and video…
The Best Diet for Depression, Nutrition Facts, 3 April 2018
The article seems to be based on his video from 2015:

… Dr. Greger argues that:

1. Inflammation contributes to depression. “People who are depressed have raised inflammatory markers, such as C-reactive protein and inflammatory illnesses are associated with greater rates of major depression. … We find depression in even more benign inflammatory conditions such as asthma and allergies.”
2. Reducing inflammation can reduce depression. (Conversely, “You can induce depression by inducing inflammation.”)
3. Diet has been shown to both promote and reduce inflammation, so,
4. Eating a diet that reduces inflammation can reduce depression.

He says that depression has probably been selected over the course of human evolution because it provides benefits, e.g. fighting infection:

Our body responds [to infection] by feeling lousy, sick, weak, tired, and slow. We don’t want to socialize. The only thing we do want to do is sleep. These symptoms are similar to the ones we experience during depression and are great for fighting infection.

There is a genetic component to depression, but also an environmental one. The predisposition to depression is genetic, i.e. some people are more prone to depression than others. In Greger’s example, the response to infection is environmental.

I think Greger’s arguments are credible. It’s not the first time I’ve heard them. However, I don’t think we can do away with depression entirely by eating an anti-inflammatory diet, nor would we want to given the benefits he described. We may be able to lessen it, reduce some symptoms, but you need the immune system’s inflammatory response, e.g. to cope with infection, cancer, insect bites, damaged tissue.

The 4 classic signs of inflammation are: heat (calor), pain (dolor), redness (rubor), swelling (tumor). Another one I learned in school is loss of function. To these we should probably add depression. All of these are useful (to a degree), albeit uncomfortable. Trying to dampen or eradicate them, say, with a pill, can be counterproductive.

On the other hand, if we’re doing something that causes inflammation and its unwelcome attributes, why not change it? This is where Greger’s argument works for me. What are we doing that’s causing inflammation? We’re eating a lot of animal foods:

The most anti-inflammatory diet is a plant-based diet, which can cut C-reactive protein levels by 30% within two weeks, perhaps because of the anti-inflammatory properties of antioxidants.
…
Where else does inflammation in our diet come from? Endotoxins. … Endotoxins in animal products can cause a burst of inflammation within hours of consumption. What does this burst do to our mood? Within a few hours of injecting endotoxins, inflammation shoots up, increasing feelings of depression and social disconnection.

There are other foods that increase levels of inflammatory compounds … vegetable oils, dairy foods, alcohol, sugar/carbs for people with insulin resistance or diabetes, saturated fat. Exercise increases inflammation also, particularly resistance training or anything that results in sore muscles. Being obese causes inflammation.