Enthusiasm requires faith. Practicing pure devotion in faith gives pleasure to the Lord. Mukunda, thinking of himself as devoid of devotion, once asked Lord Caitanya, “Why is it that when I see Your face I feel so much pleasure?” Lord Caitanya replied, “Those who have no faith in the Lord cause pain to My heart. And therefore because I feel pain, they cannot feel happiness.” That Mukunda felt such pleasure to see the Lord’s face indicates that he did have faith in Sri Caitanya Mahaprabhu. The Lord wants to give pleasure to all living entities, but when the jnanis or impersonalists say, “The Supreme is not a person,” the Lord finds their statement painful.
(LFD, Vol2, Enthusiasm for Devotional Service, Pg 36, 2nd paragraph)