He couldn't understand how one could do it. He never would.
The way his eyes rake over you as you step out of his bathroom, bare body hidden from view beneath a dark brown towel. The look in your eyes when you notice Negan staring from across the dimly lit room is wavering and the man can't help but run his tongue along his top-row of teeth with growing interest.
Of all the partners, lovers, wives and such that he's ever had, you intrigued him the most. Had the most of his attention. And it was very clear you took it to heart whenever you'd meet late in the night. You smile at him and only him. You know who you belong to in this arrangement and Negan loves to see you show it.
You were his and he was yours. And yet he found it harder than anything in the world to simply express that. To rid himself of his other vixens for your sake and tie the knot. It was the end of the world, Negan was the leader of an entire community, but nothing but himself was stopping him from indulging in your temptation for true connection.
It vexed him how anyone in the world could make that choice. What was really so good about marriage?