What do you think?

My two year old son acts completely fine when he's around me and his father. Father is a little more strict with him than I am. When his dad picks him up from daycare, he's happy to go (most of the time) but when I pick him up, he screams, throws a tantrum, doesn't want to leave. It's almost as if he hates me and doesn't want to go with me. As soon as he gets in the car, he's fine. 
Why do you think there's such a drastic change in his attitude? Am I really that bad of a mom that he wants to stay at daycare?! 
He does spend 70% of his time with me because his father, my boyfriend, works a lot.