He cannot put the box down. The 'box' is a plot device and a representation of the other 'box' that he has the key for that she accidentally touched. He's got the box in his hands and doesn't want to put it down so he can take out his key. But she will help him get his key out if she has to go digging in his pocket to grab it.
I'm thinking about this WAY too much, LMAO.