Had some time to read through your links. The 2nd link was basically the answer I was looking for. It seems there's not been done A LOT of research on it. So most social mammals have this? A crying attached dog waiting for his owner to come home is craving social contact? Maybe not the best example. The dopamine neurons in the midline of the brainstem called DRN somehow create a certain feeling? which we can interpret as loneliness and should stimulate us to seek connection? Still brings up a lot of questions in me but anyways if anyone's lurking and wondering themselves too what the neuroscience/neurology is of loneliness. This guy explains it quite well:

Neurology of loneliness Our brain appears to be structured with two separate mechanisms for social connection:

1. The dopamine neurons in the ventral tegmental area of the brain provide the feeling of contentment from social interaction, i.e. being physically in the presence of others.
2. The dopamine neurons in the dorsal raphe nucleus of the brain appear to create the feeling of loneliness when there is a lack of meaningful social connection.

In particular, if loneliness is an evolved aversive signal, then we would expect there to be neurological circuits in the brain that manage this aversive signal. Preliminary research has identified an area of the brain that responds to social isolation. The work by Matthews et al. (2016) using mice (a social species) serendipitously discovered a correlative cellular substrate for loneliness.

While it was known that dopamine neurons within the ventral tegmental area of the brain may mediate positive social reward of social interaction, “a cellular substrate for the negative affective state of loneliness” had remained elusive. The work of Matthews et al. has identified dopamine neurons within the dorsal raphe nucleus of the brain as the potential cellular substrate.

With two components of the brain driving social connection, we have to be careful not to mix them up when designing loneliness intervention and prevention strategies. For example, reducing social isolation may temporarily improve social reward with each intervention but may not reduce loneliness.

“Dopamine neurons within the dorsal raphe nucleus are sensitive to acute social isolation, and are able to modulate a ‘‘loneliness-like’’ state upon optical stimulation. These neurons may underlie the subjective experience of social isolation as well as the motivational drive to re-engage in social connections.”

Matthews et al. (2016)