It was the prophet Jeremiah who said, "The heart is deceitful above all things. And desperately wicked; Who can know it?" Nowhere is this desperate deceitfulness so evident than in matters of worship.
God is jealous for His glory, while man's heart continually clamors for his own satisfaction. The deceitful heart of man therefore deterines to gradually enhance God's worship with elements that appeal to human emotion. In doing so, God's glory is displaced by man's and the pure worship of God is tainted and ruined.