Here is a lecture by Robert Sapolsky, an extremely well-regarded researcher, that goes over some evidence on depression being a disease: https://youtu.be/NOAgplgTxfc

It's been a while since I've seen it, but I recall his evidence including some rather striking non-behavioral symptoms, like changes in sleep cycles.

To address your specific thoughts on it, his position (paraphrased from memory) is that social stressors like you describe cause elevated cortisol, which causes depression - such that your position that "modern life really sucks" is not incompatible with a biological cause.

It's extraordinarily interesting in a lot of other ways - definitely worth a watch in spite of the length.