It appears that you have a declining dogwood tree.  These trees are very susceptible to dogwood anthracnose, a fungal disease that has decimated Cornus florida trees in much of the country and may be what is affecting your tree.  Here is a link to a description of this disease and possible management:  

dogwoodanthracnose.pdf (cornell.edu)
 Management includes pruning diseased branches (during hot and dry weather) and disinfecting pruners after each cut.  Falling leaves should be raked and removed.  Water during dry periods but avoid overhead irrigation that can spread fungal spores.  Fungicides can be applied in the spring after budbreak until leaves are fully expanded but can continue if the summer is unusually wet (see above for specific agents.)
Note that it may not be possible to save an affected tree.  However, there are other types of dogwoods that are resistant to anthracnose, including the Kousa dogwoods, which are suitable replacements.
    Remember to read the label and follow all label directions when using any chemical control.