Mindfulness of the body gave us stability of focus and mindfulness of feelings gave us the mechanism for how we project ourselves onto the world. Now we are sufficiently prepared to look at the mind itself.
The "Introduction to Metta" talk positions metta as the particular form of mindfulness that reflects the third foundation of mindfulness, attention to the contents of mind. It also is presented as the practice that follows the Buddha's instructions for Wise Effort, the purposeful cultivation in the mind of wholesome states.
How do we bring a loving presence to all of our experience- especially to our humanness, failed intentions. Cultivating mindfulness & kindness with our own experience becomes the template for living a wise life.