Inactive members

In his book The Purpose Driven Church, Rick Warren counsels pastors to let go of their inactive members, saying that they left for a reason, and to spend your energy on getting new people in...and while I agree with that, it is so hard to let go. I feel that we have so much to offer, so why would people neglect their own spiritual well-being by not coming to church? If they're going elsewhere, I could live with that, but it is painful to think that these people are content to live apart from their extended spiritual family. Did they leave us because they weren't "of" us? This is the argument of the first epistle of John. I wish I knew. It hurts.