What happiness/joy God gets, when we humans pray him?
Well, certainly is that the Lord becomes satisfied and feels some joy when his devotee is doing something with devotion and affection (bhakti) for the Lord.
Lord Krishna says in the Bhagavad gita 7.17:
teṣāṁ jñānī nitya-yukta
priyo hi jñānino ’tyartham
ahaṁ sa ca mama priyaḥ
Of these, the one who is in full knowledge and who is always engaged in pure devotional service (bhakti) is the best. For I am very dear to him, and he is dear to Me.
In the verse words sa ca mama priyaḥ "he is dear to Me" indicate that the Lord feels joy and satisfaction when we do some devotional activity.
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