This story is a total change of pace for Hera... in some ways at least. Usually, the queen of gods keeps herself busy by tormenting one of her husband Zeus's lovers. She also spends a ton of time making the life of the famous hero Heracles completely miserable.
In the early days of Jason, however, we actually see Hera favoring somebody. Of course, if you dig a little deeper, you learn that the reason she favors Jason is because she really reallyhates his rival, Pelias. So, maybe Hera's favoritism of Jason is just another form of her usual nastiness.
It's also said that Hera might be a fan of Jason because she thinks he's cute. When she snuggles up to the sexy young man while crossing the river, she might just have a little more on her mind than testing the hero. If this is true, then it would be an interesting turn-around: usually it's Zeus, not his wife, who is stepping out of the marriage for a little...um...snuggle.