You and Nightmare were currently fighting. He was more amused that a human could pack a punch or fight in general. So he wanted you to join in his team. How? Negativity.

He trapped you in his tentacles of galoop. And lifted you in the air, so you were completely defendless.

"What are you gonna do now, human? Huh?" He said. Not letting you go. Expecting some kind of negativity from you towards him, or to fight back.