It's a natural feminine response. Comfort is garnered when something 'tough/masculine' acts 'soft' upon them in an unambiguously kind way.
Animemes especially, as their psyches are undiluted by post-modern sarcastics and ironies. Truthful to an absolute fault. They have no (((compulsion))) to deny their own impulses, or see darkness in your own.
Envision their soul as incense. Patting the head is like a light blow. The inner fire can't help but brighten. Be careful though, else that comfy warmth will bloom into a burning passion. Pat responsibly.