Ever since she found out that Jon is the true Targaryen heir to the throne, Daenerys has been worried about it, and has been wanting him to keep it a secret, despite him several times saying that he doesn't want it and considers her to be his queen. She also considers him telling his family of his past a "betrayal", and burns Varys alive for spreading that information. Ultimately it also seems a factor in her decision to go batshit insane during the attack on King's Landing.
Why is this such a big deal to her? If Jon doesn't want the throne, then how could this possibly hurt her? If the "people" prefer and love Jon over Daenerys (which doesn't make much sense, since Jon is only loved in the North and there are 6 other Kingdoms in Westeros that don't give a crap about him), then okay, that's not optimal, but if Jon doesn't want it and himself supports Daenerys, what's the issue then? Why doesn't Daenerys just start ruling and showing her capabilities as a ruler? What exactly does she fear? That Sansa will tape Jon to the Iron Throne and force him to be king?