Okay, on part of it, Loki could sympathize with Idun. Listening to Bragi's ballads about pure, sweet love was annoying. And predictable. But mostly annoying. At least in his opinion and for the briefest of moments he was relieved that someone else felt the same way and didn't just melt at the idea of Bragi singing about love.
Loki didn't believe in love. Idun would know that. He'd told her as much. Well, alright, that wasn't fair. He believed in love from an affectionate familial sense. He loved his mother very much and not just because Laufey was weak and frail... and sick a lot. She was unfailingly loyal and honest. She was smart and funny and didn't hold back just because someone's feelings might get hurt. Plus she knew how to put him in his place like no one else, not even Odin, could. If he thought for even a moment that another woman like his mother existed, well, then he'd marry her in a heartbeat and that would be the end of that. Loki loved his mother completely and would do anything for her. She was the greatest woman in the nine worlds and no one could argue that fact with him.
No one.
He loved his children and much like with his mother, would do anything for them. Anything, no matter what was asked.
He did not, however, believe in romantic love. It was a joke. A trick. It was something one taught their daughters to keep them chaste until marriage. It was something you taught your sons to manipulate in order to seduce other's daughters away from their chastity. No matter Frigg or Freyja's insistence on the matter, it simply didn't exist. Especially when he considered Frigg and Freyja's interpretations of romantic love.
No, what Bragi spouted was a load of nonsense, so Loki understood that it was annoying. He sympathized. But then Idun expanded on what she'd said and he realized she didn't share his opinion at all. At least, he didn't think so. For some reason she was upset that Bragi was infatuated with someone she didn't like. Why did that matter? Why was that worth getting upset over? Thor was infatuated with Sif and while Loki still thought about her daily in a far more intimate sense and had her that way in the past, he didn't get tied up in knots with Thor over it. It wasn't worth the energy required.
He returned her hug, but wasn't about to drop the subject. Obviously it was bothering his little sister a great deal and that was making it, by proxy, his problem too. “I'm glad to be here. Now...” he broke their hug to look down at her. “You're wrong, you know. Good friends to criticize.” Then he considered the word itself. “Okay, maybe not criticize. That's a bad choice of words. But you should be honest.” Loki gave Idun a firm look, “Good friends are always honest.”