Jack Heart liked to think of himself as a good man. He liked to think that such things as pettiness and jealousy were so far below him that the words couldn't possibly be part of his vocabulary.
Then he'd discovered that Alice was in Wonderland and that during her time here had been keeping company with another man.
It turned out that pettiness and jealousy were pretty good terms to use when describing his feelings towards his rival for Alice's affections.
And what kind of a stupid name was Hatter, anyway?
There wasn't much that Jack could do in regards to his rival. He was hoping that Alice would stop looking for him (even if Hatter was apparently working to help her cause, what a nuisance) so if he actually did anything it would draw her attention right to him: exactly what he didn't want. But the resistance sort of needed her to try and break through Carpenter's programming, so it wasn't as if he could nudge Alice and her not-new-boyfriend back towards the looking glass.
And then Alice managed to get herself into the House of Cards somehow, and thus she discovered that he was in Wonderland after all. So Jack tried to give her another nudge in the direction of Caterpillar (not that she knew what he was doing, that was what Stupid Hatter was supposed to figure out, wherever in the world he was larking about) and worked on slipping his way past Duchess.
He did feel a bit bad for leading Duchess on, but she was his mother's pawn. She was a strong woman, and she'd get over her hurt feelings eventually.
When Dormie showed up at Dodo's library with word that Hatter wanted to contact Caterpillar, Jack had the insane urge to kiss the man. He wasn't sure if it was Dormie he wanted to kiss for delivering the news or Hatter for having the insight to try locating Caterpillar for Alice, but it was there. And he honestly didn't like the fact that he was entrusting the safety of a woman he had come to care for very deeply to his rival.
Still, it was for the good of Wonderland, so he pushed past it.
So he had gone to the forest to meet up with Alice... and convince her that it would be for the best if Hatter and the Knight didn't accompany them to Caterpillar's hospital. It took more time to convince her than he thought it would, as she seemed very reluctant to leave her newfound friends behind, but eventually Jack prevailed through the use of doubletalk. It was probably wrong to insinuate that Hatter was not a man of honor, but... he both worked for Jack's mother and aided the resistance.
It would be foolish to trust someone who was sitting on the fence rather than choosing a side. Jack chose to ignore the fact that Hatter seemed to be acting less in a manner that benefitted him personally and more in a way that would best assist Alice.
Being found out and captured by the Spades had definitely not been part of the plan.
Nor was Hatter's ill-timed rescue attempt that resulted in his own capture.
Jack would never be positive of what happened in the final hours of his mother's rule. He'd been sprung from the Eye Room by Duchess, who confessed that she'd always loved him... and Jack found himself wondering why he'd never really noticed it. They managed to slip past the Spades and get out of the building right about the same time that Alice, an injured Hatter, and all the Other Siders who'd been captured were fleeing the collapse as well.
And when Alice refused his proposal that she stay in Wonderland as his queen, Jack wasn't terribly heartbroken. Duchess had told him that he would never be able to move on if he didn't ask, and said that if the other woman stayed she would stand aside. His fiancée's selflessness made him again wonder how much he had missed right before his eyes before.
When Hatter gave every indication that he intended to follow Alice through the looking glass, Jack only felt a brief instance of jealousy before wishing his former rival luck in his quest to win the woman's heart. He knew very well how much it took to win that particular heart, and he counted himself lucky to have held it for even a short while.
He honestly hoped that his ex-rival would be able to hold onto Alice's heart forevermore.