They go through little phases - some owners have more luck with the intensity of these phases. Usually at about 7-8 months, my little obedient puppies give me a look of..."Oh, you really cannot reach me or catch up with me if run from you..." and then I know adolescence has arrived.
Same thing can happen with their reaction to other dogs, new stimuli, etc. It can be a little surprising when these changes appear. Having a good relationship, patience, thoughtfulness and some basic obedience is key to managing phases - they become settled by 2-3 years. It's for this reason I don't like to breed until a little later - to see what the "real" adult dog is like. Like I said though, some puppies give their owners much less "hassle" than others.
