The following is from notes of my thoughts during a priesthood lesson over a year ago.
Luke 15 tells us three stories, all are about loosing a prized possession. But each teaches a very different lesson. As I recall, the priesthood lesson that brought the Spirit and helped me learn the following, quickly descended to the normal, modern-Mormon interpretations of these parables, when the teacher opened it up for commentary. It’s unfortunate that Luke wasn’t familiar with these modern interpretations when he recorded these three parables together, in what is now chapter 15 of his book.
Context
Luke sets up these parables with the context that “the Pharisees and scribes murmured, saying, [Jesus] receiveth sinners, and eateth with them.” So we must assume that what follows is Luke’s perception of Christ’s response to these accusations.

Image of a shepherd with his flock from Bible-Archaeology.info
Lost Sheep
The first story is of a lost sheep (Luke 15:3–7). If one in a flock goes missing, how does that happen? Well a shepherd can’t watch his flock 24-7, he must sleep. As he does, the flock continues grazing, moving in a direction, but generally staying in a group. If one sheep finds a particularly lush patch of grass, or some other distraction, it will become “lost” simply by staying where it is. The church can work in much the same fashion. We never want to leave people behind, yet the goal & purpose of the church is that its constantly moving forward, progressing on both an individual level as well as on the whole. Thus a person can get left behind, simply by not continuing to move forward with the group. Sins of omission still make one a sinner.