Flyleaf frontwoman Lacey Mosley grew up in a broken home and began dabbling in drugs at the age of 10. The fighting at home became so intense that she was forced to move in with her grandparents. Whilst there she experienced a nervous breakdown and considered suicide before finding salvation in her Christian faith. Much of Memento Mori is about that journey from darkness to light and about guiding others in the same way. She told MTV News: "So much of this album is inspired by the mentality of letting go and trusting that good can come out of that. About trusting that God is going to work out the details that we can't control. It took me so long to realize that, and it wasn't until I did that my life got better... so I'm hoping our fans will realize it too. A lot of our fans are kids who have so little hope in life and face so many dark things. And definitely there are times where you need to feel what they feel, cry with them, rejoice with them. But there comes a point where we have to let them go on their own. We have to hug them and say, 'We have to believe that you're going to wake up tomorrow, because if we went through so much stuff and ended up here, the possibilities are endless for you.'"
Mosley added that a couple of songs on Memento Mori are central to that theme and she was particularly thinking of her younger sister. She said: "There are definitely songs that came out in the studio, while I was in the vocal booth, that I was singing directly to my sister. Like 'Tiny Heart,' 'Set Apart This Dream,' those two songs specifically, I remember thinking of her face, singing to her, so it would come out in the most honest way. As much as we want to protect the people we love from everything bad, they have to make their own decisions. And if you're not walking beside them, they're going to make mistakes. I wanted to keep my sister from making mistakes... I wanted her to learn from my mistakes, so she didn't have to do the things I did or suffer the way I had - and there's a lot of that on the record."