It's because those words said in that context implied to both of them that:
"If it wasn't for circumstances, you wouldn't have agreed to marry me. You didn't marry me because you love me. Even though I love you".
It's not the same as "you're the best out of everyone!!" but rather, "you were the most logical choice".
hmmmm i think it really just depends on the person themselves....for example i used my own personal perspective on this: i have 2 sisters, i’m in the middle, my parents used to always compare me with my older sister with phrases like “your sister is better at this than you/she’s the best at this/you should learn from her” sometimes they would say to my older sister “you’re the most outstanding one (between us)” in front of me, not quite the same with FL’s “you’re the best choice” but it entails similar connotations
It wasn't really a perspective piece... that was a direct analysis of the story itself.
To you it might be that way, but it wasn't to the characters...y'know?
Your thoughts are valid that there could be a different way of viewing it depending on the person, but for Ercella and the Duke, it absolutely wasn't the case. They were said those words by others with hurtful intent, and then they said it to each other with hurtful intent.
The point was [what was the intent].
Why is it an insult? Or hurtful words? From the start everyone has choices and i think it's rather egotistical for anybody to think that the person has no other options aside from you. Because there's a second choice? Third choice? Instead of feeling depressed that these choices exists, should you not be happy that inspite all of those alternatives you were chosen instead and was considered to be the best???