You see, what's genius about it is that in the use of the word "we" it definetely makes you sound like a couple - "hang out" makes it sound a whole lot more casual - and "sometime" makes it open and not too rushed. Also worth note is the fact that it's not a yes or no question, it's an opinion - and you can't say no to an opinion!
The pickup line was invented - or at least popularized - by Josh Sundquist, who is an awesome YouTuber. OF AWESOME.
*guy spots girl*
*guy and girl talk*
Le Guy: We should hang out sometime.
this is something guys say just to lead you on. They don't really want to. They just say it. Sometimes not even to get into your jeans, it's often used just to be nice, even though they really don't want to hang out with you. So remember this the next time some random, or even a guy you know, says this.
You: Thats cool, I might show too.
-----------few minute silence--------
Guy: Uh, we should hang out sometime.
You: Sorry! i know what you mean when you say that, thanks to urbandictionary.com