I think there is a lot of truth to this, but I do not quite agree.
Most long-lasting negative emotions and moods exist solely for social signaling purposes
feels a bit off to me. I think I would agree with an alternate version “most long-lasting negative emotions and moods are caused by our social cognition” (I am not perfectly happy with this formulation).
In my mind the difference is that “for signalling purposes” contains an aspect of a voluntary decision (and thus blame-worthiness for the consequences), whereas my model of this dynamic is closer to “humans are kind of hard-wired to seek high-calorie food which can lead to health problems if food is in abundance”. I guess many rationalists are already sufficiently aware that much of human decision-making (necessarily) is barely conscious. But I think that especially when dealing with this topic of social cognition and self-image it is important to emphasize that some very painful failure modes are bundled with being human and that, while we should take agency in avoiding/overcoming them, we do not have the ability to choose our starting point.
On a different note:
This Ezra Klein Show interview with Rachel Aviv has impressive examples of how influential culture/memes can be for mental (and even physical) illnesses and also how difficult it is to culturally deal with this.
This time, I agree fully :)