Some dogs are just like that. My sheltie's only a year old and I've NEVER seen the puppy side of him. He does play once in a while but for a majority of the time, he's very mellow. The reason we got a sheltie was because they were some-what "active". Kai is anything but that! Lol I literally have to pull him closer to me because he's lagging so much during walks. But I wouldn't have it any other way