[Yakko]

Okay so I have to ask something I don't understand....If Sweeney never really cared that much about Mrs. Lovett, then why does he laugh and whatnot with her?

[Vichysois]

I think he's still riding on the hysterics from Epiphany. "I'm full of joy!"

[Pannic]

He might not care about her, per se, but he sees it as a great opportunity. Besides, there're enough bad puns to make a guy laugh.

[Yakko]

[Mr_X]

To him, he has one thing strictly on his mind: vengeance. As he explains in "Epiphany", he decides to take this out on innocent people until he finally gets the Judge. He thinks Mrs. Lovett's idea is brilliant, as he believes these people are getting what they deserve. So though it doesn't directly deal with his plan of vengeance, it goes along with the idea of "those below serving those up above" (like him and the Judge).

[what_the_heck013]

[Sweeney Hyde]

I find it also to be a stylistic issue... especially in the original production and those modeled after it where it's intended to be like a music hall number.

[Moxie Music]

I think Mrs Lovett is the 'final nail in the coffin'. Sweeney is already mad with anger and pain and she sees this as an opportunity to use his vulnerability for her advantage. I think he is slowly going mad. So laughing with her could be explained that way. I don't think him laughing with her is a measure of how much he cares for her.