Learn to cultivate this. Try not to sound angry when you are delighted with him. The actual words you use will mean nothing - but the tone is all important. Food, carefully used, is a great motivator for many greedy pups. But make him earn his titbits.
Some people think that it is good enough just to throw food at a dog.
It must have a meaning for the dog or it is wasted. Toys are good. Have a special toy that the pup likes and keep it only for training sessions. Produce the toy and ask him to watch. If he gives you his full attention for a second or two, break off and play.