Why is it so much harder to lose weight after the first 5 – 10 pounds?

It takes a 3500 calorie deficit to lose one pound of fat.  What that means is if you need 2000 calories a day to live and you eat 1500 calories per day it will take 7 days to lose one pound of "true" fat.  The reason the first week or two of a diet produces such bigger results is that you lose a lot of fluid (water) due to the adjustment your body is making to the lower intake of food.  Fluid loss accounts for the largest percentage of weight loss in the first couple of weeks after that you see the results of your eating program.  Just remember 3500 calories deficit for one real pound of fat loss.