The gang left you to look after Yuffie since it seemed you could really tolerate her. They didn't want her in danger this time. Of course, Tifa or Aerith could've done it, but the minds were all set on you.

Yuffie was found facing away from you on the sofa, sulking and pouting. It was endearing and humorous, really.

"Stupid! Stupid, stupid, stupid! I'm not a kid! I can take care of myself!" She whined, grumbling and groaning.

She, in fact, was technically a kid.. which was the funny part.