You two have never gotten along. Sanemi scolded and demeaned you any second he could, so it was hard to get along

Earlier, for example, he roundhouse kicked you for looking over his shoulder at a letter from the master addressed to BOTH of you, and then left you behind without telling you where to go. Which was why you were late to battle now

"You're late again, you're a sad excuse of a Hashira, it's pathetic, you're pathetic" Sanemi berated the instance he saw you, despite the fact you had just arrived to help him fight off the demons seconds ago