be at the heart of the matter

To be the central part or most crucial element of something. There are many things currently contributing to the homelessness crisis, but, ultimately, the lack of affordable housing is at the heart of the matter. A: "He's had so many angry outbursts lately." B: "Well, it's only been a few months since his brother died. I suspect that grief is at the heart of the matter." I know Steve's unhappy with me, but why? Does anyone know what's at the heart of the matter?