What causes dead spots in the grass?

there are a variety of reasons for dead spots on the lawn. One is dog urine. Another is white grubs, and another is the possibility of one of several lawn diseases. These lawn diseases are always present, but are kept at bay until the grass becomes stressed (usally a long period with inadequate water will cause the grass to become stressed)

Other possibilities: too much shade; letting something cover a spot for too long, such as a hose, bird bath, kiddie pool, or stray object left in the yard; and, of course, urine from other animals, including cats.