The best comparison would involve operating under the assumption that the hulk and Bruce Banner are a result of Dissociative Identity Disorder. In this case, Natasha acts as what is known as a 'positive trigger', something positive to Bruce that acts like magnet pulling him to 'the front'.
Because Bruce likes Natasha and trusts her, he feels comfortable around her which allows him to force the hulk back in to their mind and allow Bruce to come back out.
