She'll keep kissing him until he calms down. Slowly, gently with more tenderness than Natasha thought herself capable of. She doesn't care if anyone stares, or talks or there's some stupid office pool that gets started about them. She wants Steve to be himself again. She knows the serum doesn't make him the man he is, but it gives him the confidence he needs to be the man hiding inside him right now. Hopefully she can help him find a little of that inner strength.