Why does my boyfriend rub my hand with his thumb/fingers?

He always does it everytime we hold hands. If we're sitting and watching something while holding hands, he'll reach inside the sleeve of my shirt (I usually wear long sleeved shirts), and rub my wrist with his fingers. He'll play with my hands too and will often stare at our intertwined hands fondly with a tiny smile on his face
Why does my boyfriend rub my hand with his thumb/fingers?
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