When we had our 45kg GSD x Rottie who was reactive and always on lead, I learned quickly how many puppies would gallop joyously into her lead space to initiate play only to scream and freak out when she flipped them over (although she was always more gentle with puppies - she just had this need to flip everything first, introductions after, hence the lead!). Had she been off lead she would definitely have chased a running puppy and possibly terrified it quite severely, whereas on lead the puppy would run away and not be chased and thereby hopefully learn that maybe you shouldn't run at unfamiliar large dogs (and maybe the owners learned, too?). For me, any dog that chases when not invited, be it "playing" or otherwise, needs to be recalled and put on lead when another dog turns up. It just isn't under control otherwise and there are many reasons an unfamiliar dog shouldn't be "chased" - from training to pain issues. That said, I too don't allow my dogs to interact with unfamiliar collies as they do seem to switch into stalk mode a bit too quickly and my terriers don't take kindly to that. The adult wants to ignore and be ignored. He can brush right by a dog that's ignoring him, but he doesn't like anything interacting with him. My puppy just needs to learn (SIGH!)