SB 10.32.20

nāhaṁ tu sakhyo bhajato ’pi jantūn
 bhajāmy amīṣām anuvṛtti-vṛttaye
yathādhano labdha-dhane vinaṣṭe
 tac-cintayānyan nibhṛto na veda
Synonyms: 
na — do not; aham — I; tu — on the other hand; sakhyaḥ — O friends; bhajataḥ — worshiping; api — even; jantūn — with living beings; bhajāmi — reciprocate; amīṣām — their; anuvṛtti — propensity (for pure love); vṛttaye — in order to impel; yathā — just as; adhanaḥ — a poor man; labdha — having obtained; dhane — wealth; vinaṣṭe — and it being lost; tat — of that; cintayā — with anxious thought; anyat — anything else; nibhṛtaḥ — filled; na veda — does not know.
Translation: 
But the reason I do not immediately reciprocate the affection of living beings even when they worship Me, O gopīs, is that I want to intensify their loving devotion. They then become like a poor man who has gained some wealth and then lost it, and who thus becomes so anxious about it that he can think of nothing else.
Purport: 

Lord Kṛṣṇa states in Bhagavad-gītā, ye yathā māṁ prapadyante tāṁs tathaiva bhajāmy aham: “As people approach Me, I reciprocate with them accordingly.” Yet even if the Lord is approached by someone with devotion, to intensify the devotee’s love the Lord may not immediately reciprocate fully. In fact, the Lord is truly reciprocating. After all, a sincere devotee always prays to the Lord, “Please help me to love You purely.” Therefore the Lord’s so-called neglect is actually the fulfillment of the devotee’s prayer. Lord Kṛṣṇa intensifies our love for Him by apparently separating Himself from us, and the result is that we achieve what we really wanted and prayed for: intense love for the Absolute Truth, Kṛṣṇa. Thus Lord Kṛṣṇa’s apparent negligence is actually His thoughtful reciprocation and the fulfillment of our deepest and purest desire.

According to the ācāryas, as Lord Kṛṣṇa began to speak this verse the gopīs looked at one another with squinting eyes, trying to hide the smiles breaking out on their faces. Even as Lord Kṛṣṇa was speaking, the gopīs had begun to realize that He was bringing them to the highest perfection of loving service.