Why does your 8 month old boy puppy get aroused when you pet him?

An 8-month-old male puppy is the equivalent of a teenaged boy. He has recently become sexually mature, and his body is very sensitive to physical contact. When you stroke him, he feels pleasure, which can translate into sexual arousal. This is true whether he is intact or neutered. In addition, male dogs in general achieve erection very easily because of the structure of the canine penis.

Note: If your puppy is still intact, you may wish to consider having him neutered, unless he is a champion show dog and you want to breed him.