Because they probably aren’t self-righteous dingleheads. In my experience, as soon as you start thinking that you’re spiritually ace, you’re not. It’s an easy trap to fall into because we all have this inclination to worry about how faithful we appear to others, and as soon as we begin to pay more attention to keeping up appearances (which is easy) instead of keeping up our relationship with God (which isn’t so easy), we become like the Pharisees:
For they preach, but do not practice. They tie up heavy burdens, hard to bear, and lay them on people’s shoulders, but they themselves are not willing to move them with their finger. They do all their deeds to be seen by others.
— Matthew 23:3-5 (ESV)
Paul had some sound advice. To the Galatians he wrote:
Brothers and sisters, if anyone is caught in any transgression, you who are spiritual should restore him in a spirit of gentleness (but!) Keep watch on yourself, lest you too be tempted. Bear one another’s burdens, and so fulfill the law of Christ. For if anyone thinks he is something, when he is nothing, he deceives himself. But let each one test his own work, and then his reason to boast will be in himself alone and not in his neighbor. For each will have to bear his own load.
— Galatians 6:1-5 (ESV)
Don’t worry too much about your hand-me-down faith, the faith that has seen a lot of wear and tear and fret and worry. That kind of faith is strong, even when you are weak. When you stop worrying about your faith, when you want to take it out of its velvet-lined box and show it to guests, when you think ‘Hey, mine’s shinier than hers‘, that’s when you’ve got to worry.