Sunshine moves her hand, gently resting her warm palm against his cheek, her fingertips brushing against the line of his jaw. "You must?"

"... I doubt you'd suffer anyone but yourself to decide what you must or must not do," she says, gentle, but ever-so-slightly dry. "You 'must' do this only because you decide so. There is nothing forcing this... turmoil in you but you."

He has the will to do what he wants. Whatever he sets his mind to. Saying he 'must' removes all sense of agency on his part; he becomes a passive participant in some situation beyond his control.

And if not in his, then whose?