Do you have time?
The biggest time commitment in owning a dog is in the form of training and exercise. All dogs need training in order that they can function in a human society without being a complete nuisance. This means a regular daily commitment of ten to twenty minutes from you, in addition to your regular interaction with the dog. Training cannot be saved up for the weekend, your dog will have forgotten most of what he has learned the weekend before, and he does not have the attention span to concentrate on you for an hour and a half.
Exercise is required on a regular basis. For some breeds of dog this means at least an hour a day of walking or jogging to keep your dog fit and healthy. While your dog will not come to any harm if you miss a day of exercise occasionally, a daily routine is really the only way to ensure that you build this important habit.