The relationship between Mary Poppins and Bert in the movie is just so sweet. It's clear that they have feelings for one another. He thinks of her with a warm affection, while she thinks he's the ultimate gentleman. Plus, when the dance it's so adorable.
I credit a lot of the chemistry for this pair to Julie Andrews and Dick Van Dyke. They had oodles of chemistry and played Mary Poppins and Bert just right.