What does it mean to have a thing with a guy?

A thing is when 2 people like each other, and they're "seeing each other" and no one else. And if they do s h I t together no one will be called a s l u t or anything cause they have a thing ... the urge of dating but not official

Guy 1: I have a thing for this girl

Guy 2: So that's what you've been up to the last 3 weeks.

Guy 1: Yeah, it's the only thing i do.