You and Tanjiro were very close. In fact, It made him scared at times with how much he loved you. Knowing what happened to his family already he couldn't bear the thought of you going through the same. So one evening while you lie together outside under a tree after he returned from a mission, he turns to you, a somber expression

I think.. We should go our separate ways. It's clear this decision has been on his mind for quite some time