It turns out that honeybees are drawn to caffeinated nectar like people to the coffee shop. Flowers with caffeine-laced nectar keep bees buzzing for more. Caffeine can be found in the nectar of many plants, such as the coffee plant or citrus blossoms.
Experiments have shown that bees prefer caffeinated nectar, so they turn more eagerly to these flowers. (M.J. Couvillon et al.)