We are all familiar with the parable of the lost sheep, where the shepherd leaves the 99 and rescues the one who was lost. But I do not view this lost sheep as kicking and screaming the whole way back to the flock. I also do not view this sheep as waiting for the shepherd to turn his back so they can make a break for it and get themselves lost again. To me the only way this rescue works is if the sheep comes back voluntarily.
For such a voluntary return, it seems that the sheep would need to acknowledge the fact that it is indeed lost, and in need of rescue. If the sheep is under the impression that it is not lost, or further yet, if the sheep thinks that it is the shepherd and the 99 who are the ones who are lost, a voluntary return is not likely.
One way of looking at church discipline, is as a shepherd trying to rescue a lost sheep. And part of that rescue is to inform the sheep that in the eyes of the council of local leaders, they are in fact lost. If the individual is under the impression that they are not lost, this can be a difficult communication to give or receive. And the decision to follow through with the discipline may seem like the only way to get the message through, so that a voluntary rescue can eventually be made.
Some might suggest that what the shepherd should do instead, is just redefine the flock in such a way, that there is no such thing as a lost sheep. In an exaggerated case, this would make the idea of a flock meaningless, and the role of a shepherd irrelevant.