My brother isn't dangerous
Some people expressed concern at the last comic that Sans might get hurt roughhousing with his brother since he only has 1 hp. Sans would be perfectly safe! Papyrus is the only monster in the game who will stop mid-attack the second he notices you’re at 1 hp. Not even Toriel can do that! Which tells us that a) Papyrus is extremely good at telling when people have reached their limit/are about to collapse, even if they’re relative strangers and b) he has absolute complete control over his magic (he can even do zero damage if your hp is too low) and can stop instantly if he feels like it’s getting too dangerous.
So he’d actually be the safest and most ideal person Sans could ever tussle with. In fact he might be the ONLY person because anyone else would be too risky. Learning how to playfight with someone as fragile as Sans could even be why Papyrus is so incredibly careful in battle in the first place.
(It could also explain why their attacks are so similar at points.)
Edit - Check out Deathstar 813's dub!
Sans, Papyrus (c) Font Skeleton Hell- I mean, Undertale