Merlin thinks Snape is awesome. I mean, he teaches other wizards!! He can make potions!! That is really amazing and totally beyond Merlin's world-view of magic. He cannot really comprehend it but it sounds like a dream come true. He also gets that Snape is a little snarky and is probably easily annoyed so he doesn't want to step on his toes. Because Snape is cool and Merlin really wants Snape to like him back too.