In order for the ending of Frozen to work, the audience has to believe that Hans never truly loved Anna, and to be fair, he says as much in his villainous reveal.
But until his admission, the romance between Hans and Anna is never once presented as anything but utterly sincere and is consolidated in the most earnest way known to man or beast: lyrically. Anna and Hans' duet opens up a whole new wor- sorry, a door to the future. Theirs is an instant, wonderful connection we have no reason to doubt:
For crying out loud, those white gloves are spotless!
Is Hans despicable enough to lie through a spontaneous musical number? Well, here's the thing: Nobody lies in Disney musical numbers. Not really. If you're a secret villain, you drop conspiratorial winks faster than trousers on prom night -- in both cases thanks to stirring ballads. For example, when The Little Mermaid's Ursula sways Ariel into a Faustian bargain, she can't resist giving sneering asides to her minions, and Tangled's Gothel can't even song-convince her daughter of her beneficence without a full verse dedicated to calling Rapunzel fat. A villain can't help but reveal their true douchiness through song. Hans can no more hide his heart in harmony than he could from an omniscient narrator.
According to his song's lyrics, the closest thing Anna has to a flaw is that he didn't meet her years ago. And that's important, because ...