>>1927
Because material existance is suffering. It's not that there's only pain, but that the inevitable fluctuations between happiness and distress cause deep rooted anxiety in the living entity. You do not really enjoy life, you temporarily experience a limited material enjoyment as a result of your activities (karma). But each and every living being seeks joy because that is the very natural state of spirit, it's an eternal attribute of spirits which we've forgotten due to attachments to ahamkara (artificial/false self).
Now, I do not agree with the person who you replied to because there's a fundamental flaw to the new age way people talk about "merging with God." It essentially means that we're God in illusion, which presents a logical impossibility. God is an eternal infinite being, the Absolute Complete One, who is the source of all that is. Nothing can "become" God because God is forever stable by aforementioned axiom; He just Is. Another flaw is that if we're God who's fallen down, then God cannot be Absolute and illusion is superior (meaning illusion has to be God, but it cannot be because it has no self-sustaining existence).
Instead, we become united with God. Everything that exists belongs to either three ontological states. Starting with the Highest, one is Brahman, which is the Absolute. The second is Atman, which is eternal spirit consciousness (us). Ending with the lowest, third is jagat, inert