>>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 materiality. We belong to the second category in our purest experience, but are now mixed with jagat due to illusion. This mixing induces suffering due to an artificial mixing of atman and jagat, where temporality is intruduced due to your limited nature: we're eternal but not infinitely expansive, meaning our creative endeavours with jagat are can only ever be temporal which is contradictory to our essential nature. Both Atman and Jagat are qualities of Brahman, each serving a different function. One may see this in the example of the clay pot. The clay and the pot are one, but without the clay the pot couldn't exist, but the clay could exist without the pot. They are one in that they have completely linked, but we see a difference between the clay and the pot functionally and qualitatively. Uniting with God works similarly in that we are eternally linked to God, and dependent on Him, and yet we have a different function. The unity is a matter of relationship, as opposed to the pot becoming the clay (self-annihilation). One can technically do that by merging with the impersonal light of God reverting to a general awareness, but the Atman exists due to Gods will. To try and merge with God, either in the original Advaita sense (self-annihilation) or new age sense (one becoming God), whatever way you view it you are denying God's wants. And due to the essential nature of Atman, one will at some point either choose materiality again to explore a false self, or go to Godhead and be true.
Also I should add, if we were once fully liberated and one with God but as separate beings qualitatively, and if this is what the previous poster meant, then that's still problematic. If we are fully liberated, then by definition we cannot fall into illusion. So to say we "again" become one with God is inherently contradictory. One reason is because if we accepted that we again become one with God, we also have to accept that we can fall down again, infinitely many times. Thus, liberation cannot be liberation and one cannot become one with God.
TL;DR If you want to experience your true self and never-ending bliss, peace, contentment, and happiness, you will want to escape limited temporal "happiness" and know God instead.