Sam hates how the praise does something for him on a deep level. It's not just the omega instinct preening at it - it's also Sam himself. Part of him going 'yeah, I do take knot so well, thanks for noticing'. Bucky doesn't need to know this, of course. His praise isn't welcome, and Sam finds him annoying.

Please, daddy lies on the tip of his tongue, but Sam bites down hard on it. Won't give Bucky the satisfaction. Just cries, over sensitive and unable to escape the harsh pace Bucky's setting. It feels so good, and Sam wants to hate that, too.

"Please choke on your own dick, alpha," Sam snarls instead, and hopes it doesn't give Bucky ideas for what to do with Sam's mouth, actually. "Ain't gonna beg for your cum."