Humans are a social species, and quality of relationships is consistently shown to correlate with mental health.

I've seen that in some cases the definition of mental health will explicitly score against things like "lacks close relationships" or "does not seek companionship". So it always seems to me a bit circular to just assert "being more social is more mentally healthy" when the definition of mental health bakes in "being very social".

If I were to define mental health to include "desires and enjoys spending lengths of time in solitude", then I could assert "Humans as a species crave solitude, mental health is shown to directly correlate with the drive and ability to be alone."