Be Careful What You Wish For: Knowing that placating Isabela's request will only end badly, Hiccup decides to help her break up with Mariano because she won't learn otherwise.
...for some reason, he felt a very strong impulse to stick it to her. ...By doing exactly what she wanted. It was an odd impulse.
Chronic Hero Syndrome: Invoked. Isabela tries to butter Hiccup up about him being a hero and therefor helps people in need before asking him to break up with Mariano for her.
Comically Missing the Point: When Dolores expresses interest in Mariano being single, this little exchange happens:
Mirabel: Oh my gosh! Hiccup, did you see that? Hiccup: Yeah, I did... how does she manage to shuffle away so fast with such a long skirt?
Hates Being Nicknamed: When Hiccup offhandedly calls Isabela "Izzy", she sneers at him for it.
I Can't Do This by Myself: While Alma offered to do it for her, Isabela came to realize that she can't bring herself to break off the engagement with Mariano because of how awkward it would be. She ends up asking Hiccup to act as messenger.
It's All My Fault: When Hiccup breaks off the engagement for Isabela, Mariano is overcome with grief, blaming himself in his complicity with their engagement.
The Matchmaker: The moment she realizes that Dolores likes Mariano, Mirabel conspires to push the two together, dragging Hiccup along to help her. While Hiccup is apprehensive at the thought of continuing to mess with the love-lives of those around him, he caves and helps anyway.
Mirabel: As her cousin, and her cousin's best friend, it is our solemn duty to ensure true love prevails! Hiccup: Or maybe we could just let nature take its course and let them figure it out themselves? Mirabel: Pfft, come on Hiccup! We're not gonna get too involved! We'll just... nudge them in the right direction!
Proxy Breakup: Isabela asks Hiccup to break off her engagement with Mariano for her, having promised to do so but putting it off because of how awkward it was. This results in a distraught Mariano blaming himself for emotionally pressuring her.
