The best reason that I can come up with is pride, but if you've got a different or better idea I'm all ears.
Whose pride?
The post itself is plainly intended to provoke.
The subsequent comment regarding Mormons and Jehovah's witnesses is plainly intended to insult.
I, for one, will not bother.
As funny as it sounds, I think the reason is love. We all wish to share the love Christ has given to us with our brothers and sisters and help them to see His light in the same fullness as He has shown it to us. The problem is that love so easily turns to hate and resentment as we each seek to defend His Truth from what we see as its debasement. Hell's greatest victory might well have been scattering Christ's followers and using their love of Christ to turn them against one another. I truly long for the day when all of Christ's followers will be free from denomination.