Cameron and Dave are friends. They are talking about a party.
Cameron: I'm so excited about our high school party!
Dave: Really? Why?
Cameron: I asked Julie out and she said that she'd be my date.
Dave: Hmm.. that's cool!
Cameron: I don't even see the point of you guys going out together. It's not like you are going to stay together or anything. You are going to two different colleges in two different states.
Dave: Man! Sometimes you are such a Debbie Downer.
Cameron: I'm just being realistic.
Dave: Actually, you are just being negative.
Melanie and Valerie and are co-workers
Melanie: I've worked at the flower shop for 5 years.
Valerie: Do you like it?
Melanie: It's okay.
Valerie: Just okay? We have such amazing benefits and our boss is so nice.
Melanie: Yeah, but sometimes the AC doesn't work and we don't have a place to park our cars.
Valerie: Well don't be a Debbie Downer and stop looking at just the negatives. Think of all the amazing positive things about the job!
Meaning - Someone who says something negative and ruins a positive environment. This idiom comes from Rachel Dratch's character, Debbie Downer, on Saturday Night Live.