It could have been amusing at first. But, then it wasn’t, because despite the fact Sally was doing it again, talking about him in such a general sense; she really could have meant anyone, but it was still so obvious she meant him, despite all that - she was still comparing him as the next big downward spiral in her life. And Justin, didn't want the credit for that.
"Having someone apart of your life, isn't always a downward spiral, Sally." Justin, felt the need to defend himself, even if they were still talking in general terms. Because, they did that. It almost made things less complicated. It made it so, there was always a back up, a get our of jail free card. So that one of them could; whenever it was getting too real for them, say they didn't mean that. Then the other person was the one looking stupid and scarily honest.