[He may not have experienced his own anger, but he definitely felt it watching the memory, and he can sympathize with...himself(????) on how insistent he was to shut down the portal. Every sign pointed to Extremely Bad Consequences. So the fact that she doesn't know what she would have done worries and kind of terrifies him.]

It's not about right and wrong! Think about it, Mabel -- if you'd let the portal activate, you could destroy the world. If you didn't let the portal activate, what's the worst that could happen? Stan gets disappointed? Is that really worth more than the entire planet?

[Maybe this doesn't matter considering they're nowhere near the portal physically (and in Dipper's case, chronologically), but it's suddenly very important to him that he gets this point across.]