Besides serotonin, it also dumps adrenalin for me. Most of those videos make me feel watched, like my personal space is invaded and keep me on the edge. If there's no intelligible sounds in there, I can use it to help me through the afternoon gap, where my body goes into standby mode, if I don't keep moving.
I think one theory is that part of the reaction (that to shrill noises) comes from an old mammal reaction to the cry of birds on the hunt, that genetics haven't quite got rid off yet. The other part probably is due to certain effects of intimacy created by the bin-aural recordings, which place the virtual source of the audio close to the head.
There's also some research into the musical chills, which according to some research intensify for performers, the more they practice a certain piece (heard an interesting lecture on that, a few years before ASMR became a thing). At that time, the mechanism was still not understood, but the study of such physiological reactions is real, if restricted to what appears to be a small community.