Some might think that showing depressed people some cute dog pics would lighten them up, but that isn't how it works. On the contrary, the expectations towards themselves to be happy can be counterproductive.
I believe this is the crux of the matter, since social media tends to load people with expectations, depression or not and advertisers try to use this handle to promoted their products.
If a site would try to "transfer positive emotions" I would be seriously concerned. Humans don't even know what content would be best for an individual. How would you train an AI? From resonance of other users? No, that leaves out a lot of context.
But I still don't think AI curators should be regulated, because the quality of regulation would certainly be horrible.
You cannot avoid the machine entirely, and all it might take is showing a thumbnail or a preview of something to you.
This sort of thing has led me to getting off those services.
On the other side the platform could transfer positive emotions. The issue I see is that we have unregulated emotional and potentially addiction transfers directly to consumers brains. What I mean for cigarettes there is warning here there is nothing but the product is probably as addictive minus health effects.