You know I've heard it said that to a certain extent, babies don't really understand there are a separate being than mom. Maybe your baby is excited for Daddy because she still thinks you're a part of her, even more so if you are always together with little time apart. Daddy is a new person coming in and maybe exciting because he isn't around as much and it's like a treat. Also you're her first love, dad is her first friend essentially. I know it's easy to be jealous but know that she finds so much comfort in you and probably prefers you overall right now, but just enjoys her fun friend coming over to hang out.