When we lose one we love, our bitterest tears are called forth by the memory of hours when we loved not enough.

This quote is talking about the sorrow and regret of not having spent enough time with a loved one after they have passed away. It literally means that the most painful tears come from reflecting on moments spent with the person when the time was not cherished or fully appreciated. It is a way of saying that once the person is gone, it can be heartbreaking to think of all the moments in which we could have loved them more and spent more time together.