In this case, eating seconds is not a self-destructive act but an example of cognitive dissonance. Maybe it's not true for everyone, but most have the feeling that they're doing something wrong/unhealthy when they indulge in too much dessert, but they have the temptation to do it anyways.

That's cognitive dissonance. Self-destructive behavior is just one example of a weird way we deal with that tension (in this example, maybe that's the person who says "I have bad genetics, so it doesn't even matter anyways!" before stuffing their face).