I suspect that like most things, oxytocin interacts in a complex way with other chemicals in the body and that it is affected also by what we do with our bodies and how we interpret the world around us.
Rises in oxytocin are also associated with stroking pets. Dogs also experience rises in oxytocin when interacting with humans (and probably other dogs too).
Probably the scientific speak for "fuck if we know" is right about it.
Candace Pert writes some good stuff about neurotransmitters - particularly about the influence of the mind and the body on each other. Her books are written for lay people so quite easy to follow.
Here's a youtube video: http://www.youtube.com/watch?v=pJBUmdXxsSg
I also rather like Bruce Lipton's book Biology of Belief
which covers some epigenetics - so more about how the environment around us can influence the way our bodies work.