Just reading the dialogue, it's hard to understand what they're saying, but in context of what they're talking about in the show, we understand the meaning. Monica is dating a guy her friends really like, but she isn't feeling like she really likes him. So, "feel the thing" is referring to the emotions, passion, or magic that everyone should feel in a relationship. She doesn't feel any connection with the guy, but her friends do meaning "they feel the thing". Paula says that in a relationship "you should always feel the thing", or feel a connection. Hope that helps!