Flowering dogwoods, unfortunately, are susceptible to several diseases and pest issues that can contribute to their decline. Powdery mildew is a common problem and can cause leaves to have a droopy wilted appearance. Dogwood borers can get into the bark of stressed dogwood trees and their feeding damage can lead to dieback. Take a look at our publication about dogwoods and the common problems/causes. https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs...
Dogwoods also do not tolerate poorly drained soils or heat stress. The excessive wet weather last year and into this spring has been tough on many plants. If the tree is very important to you, you have the option to have it evaluated on-site by a certified arborist. They have the training and tools to test for diseases/pests/structural issues and can make recommendations for you. You can find a certified arborist using this website from the International Society of Arboriculture. http://www.treesaregood.org/