In "Mhysa", I came up with a theory why Ygritte shot Jon. Haven't read the books!
In the previous episode, you book readers knew that she actually didn't defend Jon at the Gift in the books, but she did in the series*. When Jon left her just randomly, she maybe felt like betrayed and heart-broken as if he didn't love her. And now, I think the real reason she shot Jon is because he was the only one she had ever truly loved ("I'm yours, and you are mine") and she was afraid that she would never see him again.
So, according to my theory, I think the reason she shot Jon is because of their love for each other.
What is your opinion about this? And do you have any other suggestions why she shot Jon?
(please don't come with any future spoilers from the books, thanks!)