Because it's easier to point the flaw to the other person than to accept accountability for their role. To me, it's a quasi flag because it does take 2 to make or break a relationship. It's also a flag because it umbrellas into "poor victim me". Feel sorry for me before learning who I am. It's another flag to because it raises question about their ego.

I clipped out the second questions and now wish I'd left them because it's a positive thing to ask what their role was, despite the question being a bit debateable because those that cheat do have free will and choices to make.