When Creon, king of Corinth issues the decree of exile against Medea, Jason comes to her and attempts to convince her to change her mind by seeking the pardon of the king and agreeing to his terms.
Jason has no intention of going into exile with her.
Jason argues that it would be to the benefit of everyone: to her, their children, and to himself (most of all) if she would accept his marriage into the royal family of Corinth, it would give them a position of security, in a place of strength and power.
He asks Medea to trust his judgement, believing that his judgment has proven to be of benefit to her in the past, which of course is a misreading of their earlier adventures.
Jason says to her:
In the first place, you have your home in Greece, instead of in a barbarian land. You have learned the blessing of law and justice, instead of the caprice of the strong. And all of the Greeks have realized your wisdom, and you have won great fame. If you had been living on the edge of the earth no one would ever have heard of you…I shall demonstrate, one, my wisdom; two, my rightness; three, my great service of love to you and my children…What greater windfall could I have hit upon, I an exile, than a marriage with a king's daughter? …What I have wanted, first and foremost was a good home where we would lack for nothing (well I know that the poor man is shunned and avoided by all his friends); and secondly, I wanted to bring up the children in a style worthy of my house, and, begetting other children to be brothers to the children born of you, to bring them all together and thus unite the families.[i]
Medea does not relent. She listens to Jason's rationale, but she is not fooled by his clever argument. She looks into the future and she sees a life for herself as a second wife, something akin to chattel, a war prize brought back to Greece from his adventures abroad, nothing more than a trophy. She sees her children growing up in opposition to the children of the Glauce, and rightly views the princess as a threat to both her station, and her offspring.
In keeping with the times, Medea is right to presume that the Glauce would likely have them killed to prevent them from usurping the throne of Corinth.
It is not possible for Medea to view such a future as a boon or a benefit to either her or her children, in fact she sees it as the path to their destruction, as such it is not possible for her to agree with Jason that her life is better off, now, among the Greeks than it was before she left Colchis.
In Medea's perspective she has not met justice, nor received the benefits of law. In fact, she is being strong armed out of town because the king wants to wed his daughter to the heroic Jason, and Jason self-servingly wants the position.
The situation Medea is in, is the exact opposite of what Jason describes. He is deluded, and he is attempting to delude her. She is being treated lawlessly, according to the caprice of the strong.