Good morning, Aphids, a small insect, is likely causing the leaves to curl. Cold temperatures during the leaf budding period could also cause leaves to curl. Since our weather has not been unseasonably cold this spring, it's likely aphids are the problem. Look under and around the stem and leaves and you ought to see these small soft bodied insects -- they can be red, yellow, green, brown, or shades in-between! If they aren't too many, you can wash them off with a jet stream of water. You can also use something like a neem oil or other insecticidal soap both of which can be found in a garden shop.

Below are a couple of articles that provide more information on aphids and treatment options.  

How to Grow and Care for Spicebush (thespruce.com)
Common Insects and Associated Pests Attacking Bedding Plants and Perennials | Ohioline (osu.edu)

Regards,