Charles Spurgeon Commentary 1 Chronicles 21:3

"And Joab said, Jehovah make his people a hundred times as many as they are: but, my lord the king, are they not all my lord`s servants? why doth my lord require this thing? why will he be a cause of guilt unto Israel?" — 1 Chronicles 21:3 (ASV)

It greatly magnifies a wrong action when we are rebuked for it, especially if that rebuke comes from a man not known for having a tender conscience, who, despite his roughness (like Joab), nevertheless remonstrates with you: “Why do this?” The people generally understood that when they were numbered, it was for the purpose of taxing them and demonstrating David's sovereignty over them. However, David was not their sovereign; the Lord God was their King. David was merely the viceroy, and when he began to count them as if they were his own, it became a source of great indignation to the Most High.

I am afraid that when you and I begin to count up what we have done, or to calculate how much we have given or accomplished for God, we start to take a measure of glory for ourselves. We would do better to leave that alone, for although pride may not seem like a great sin in human estimation, it is assuredly what brings the utmost wrath from the Most High. He cannot endure pride, especially in those whom He has lifted up. He took David from the sheepfold, and if David has now become great, David must be brought down again.