I've heard that it is supposed to be "less demanding" (or something in that regard) to ask a negative question when you're asking for a favor or anything alike.
Let's say I want to turn this sentence into a negative one:
スカイプで話しましょうか?
How do I go about doing it? I want to convey the following meaning: "Should we talk over skype?"
Do we talk on Skype?
But in common usage, people make that less direct by negating it, or making it voluntary &c.:Can we talk...
,Would you like to talk...
,How about talking on...
,Couldn't we talk...
,Do you wanna talk...
, and more.