Dogs do pleasure growls as well as warning/aggressive growls. You can usually hear the difference quite distinctively, hard to explain but it has a different tone to it and obviously you also have all the body language, so my view is that a dog who is really excited running around the house with a biscuit and growling to herself, that's just pure excitement and happiness. If I throw a stick into the river for Bella she will growl as she runs into the water to get it because she just loves it. She also growls doing the wall of death round the house when she's just over-excited.
Bella and Zorro continually play growl at each other during games, their noises range from little squeaks which sound a bit like chimpanzees, to pleasurable little rumbling noises to slightly more boisterous growling - none of it's aggressive, half the time they literally lie there cuddling and nuzzling each other with their tails going like the clappers - they couldn't be happier!
