Imo, its becuz aiden has continuously lied and not told wendy about anythin important about him or the things that they agreed to. Important things that could literally change her life that she worked so hard to have (that means her being one of the best witches and all of her achievements as of now as a human and a witch). So now, wendy (a very upright and out spoken character) is at the risk of having many things shes worked hard for, in her human life, become sort of tainted(?) or messed with bcuz her life is literally tied with a completely different species life. Who, keep in mind, is far stronger and more capable than her. So she would never really be able to win against aiden in terms of magic or physical strength, since hes an almighty dragon. He has the power to completey dominate her. And thats prolly scary to her too.
It's so cute idk why people are hating on this