Yeah I have to admit Jonathan going back for the dog was kinda silly and contrived. I thought for sure he was going back to get more people that he saw, but for some reason it ended up just being... a dog stuck in a car? Huh?
Still though, the final look between him and Clark was so powerful and emotional that I thought it made it totally worth it in the end.
I would try and save my dog!! My little buddy is a great friend so it makes Jonathan's actions very plausible.