This was one of my theories first. But after he asked her all those questions obviously kinda nervous what ahe will answer, especially after he asked “would you live for me“ I think he’s just preparing himself for the jump into the chemicals. This is the place where he, the Joker, was born. And I think this is a very painful memory for him. He needs a moment to get the strenght to go through this again. But as he said: He would do anything for Harley ♥