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What is the reason for him to do anything? In fact, there is nothing He needs to do.

In fact, there is nothing He needs to do.

Yeah, sure! He has no duties. But He's acting to get more pleasure. In the Vedanta-sutra you'll find the statement that God is immersed in pleasure actions all the time - anandamayo 'bhyasat. So the reason for Him to act is to get more pleasure.