This may be the dumbest question ever... but I notice that if i go more than a few weeks without having an interaction, I start getting depressed and feeling tight. If I get some, I feel noticeably calmer and more confident/at ease for at least a week or more. Why is this, besides the obvious answer?
I'd say that evolution would not have worked out anything more detailed, as long as the basic mechanisms worked for its purposes... so e.g. "doing it without an actual partner" might not change much, as long as the reward mechanism gets enough people to do it with a partner...
(Of course I'm attributing intent here, but it's a random process + feedback)
