Taking ourselves as an example, we're frankly not surprised when our girlfriend shows her annoyance. Day in and day out, we do some pretty dumb stuff. We're surprised she's still with us, to be honest. There are times, though that she'll get mad at you for no reason. When something tiny that she used to let slide suddenly turns into a catastrophe.
You know why she's doing that? It's not just that she hates the way you eat an apple (by the way, you're not, repeat not, meant to lick it all over first), it's just that she's finding any excuse to hate you. And when you're cheating on someone, hating them is far easier on your guilty conscience than feeling sorry for them.