The tendency to relish the unsurpassed beauty of Krsna’s form is yavadasraya-vrtti, another name for which is mahabhava. Krsna is Rasaraja, and the gopîs are mahabhava, and when they meet, the gopis relish He who is the very embodiment of rasa. But the dynamic nature of this meeting is that sometimes Krsna becomes yavadasraya-vrtti, and the gopis become rasaraja. This is not possible for anyone else but the gopis; being overpowered by their prema, Krsna begins serving and attending them.
Suppose there is a young boy who is lame, ugly, and unable to speak properly. It is doubtful that others will be very affectionate towards him, but his mother will always love him. Why? Because she possesses ‘anuraga’, spontaneous love for him, and although that is simply a reflection of the genuine sentiment, anuraga is the root cause of love. Because the gopis’ anuraga for Krsna expands unlimitedly, it is called yavadasraya-vrtti. Upon receiving Krsna’s darsana, the gopis’ prema multiplies millions of times—this is the vrtti, tendency, of their anuraga. But this only occurs when they see the unsurpassed madhurya of Krsna’s form; it does not happen when they see anyone else, including Uddhava, who looks so much like Krsna.
Bhakti Rasayana page 99 [CC-by-ND GVP]