Common Reason for Dry Cookie Dough #2: Wet Ingredients Not At Room Temperature
When adding in your melted butter, brown sugar, white sugar, or eggs, or any extra ingredients that add liquid to your homemade cookies, make sure that you've allowed them to come to room temperature.
Common Reason for Dry Cookie Dough #3: Not Enough Fat or Enough Liquid
Be sure to use the full amount of butter, oil, milk, or eggs that the recipe calls for. Also, be sure to use large eggs if the recipe calls for it -- smaller eggs contain less liquid.