Squee! I ship Jack/Elsa because not only does their relationship make thematic sense, it's believable and much more so than Jack Frost with Rapunzel. Jack and Elsa have sisters they would spend eternity alone for. Elsa would definitely learn to let loose with Jack around. She doesn't want to be left alone. She ran away because she had no other choice, because she wasn't understood, and because she didn't want to harm anyone with her powers. Unless you've been in their shoes before, how do you help someone the most? Enter Jack Frost, who can help Elsa realize the error of her ways by reminding her how to have fun again by being playful with her and Elsa retaliating by just being herself. Their personalities complement the other nicely so this works well without a flaw. It's a match made in Heaven. Jack is also older than Elsa by being immortal.