When we are in the park, my dog usually goes hurtling towards other dogs but when they show an interest in him he is scared and runs away. He has done this with every dog he's met since we got him 3 weeks ago.

But yesterday when we were out for a walk, he ran up to another dog and wouldn't leave it alone, and then when the other dog ran off home, my dog started wimpering and pining after that dog.

Can anybody tell me why he was like this with this dog? My dog is a Cavalier King Charles and the other dog was a big black dog (I'm not very good at guessing breeds yet). And yes, they are both dogs, not bitches.