Blue crowned conure bird behavior?

My father's conure is only nice to him and him only, and will proceed to rip off of anyone elses fingers. However there was one day I remember he was perfectly behaved around me, sitting on my finger, and snuggling on my shoulder, around my father's presence. Why does he act like this? He sees me as a threat unless he is ped to me, otherwise he will attempt to lunge at my hand.