Jyn wants Cassian to face up to the consequences of his actions
At this point, Jyn Erso has come to know that Cassian Andor is well and truly a 'spy'; in every sense of the word. He has shown that he can talk his way out and around any problem. She doesn't trust him, but wants his honesty.
The context of this scenario is that Cassian went against his word to Jyn about the mission to 'rescue' Jyn's father, Galen Erso. When she realises this, Jyn confronts Cassian and makes him account for it.
"You told me we were rescuing my dad, by you went to kill him instead, now he's dead!"
Cassian is not at the point where he feels like he does need to explain his actions to Jyn, though. Besides, he was given a very strict order from his superior not to save Galen. Who's Jyn to question him?
Interestingly, you can see the pain in Cassian's face when he has to go through the whole ordeal. I'm sure he was thinking in his mind that he just wants to comfort Jyn and tell her that he had changed his mind. But given the pressure he was under, he doubled-down and told her to get off his back.
"The mission is bigger than you or I, and I'm happy with my choices. I don't have to explain them to you..."