He didn't want to be alone for eternity? If that the case, then what good would destroying the galaxy do? If Starlord refused to join him (which is what happened), then he'd have to find another female to breed with to create another successful offspring. But, wouldn't this be impossible if the galaxy was destroyed? It just didn't make sense to me.
shareAt first he didn't want to be alone but mortals disappointed him, mostly since they didn't carry Celestial genes, so he decided to absorb the entire galaxy using whatever celestial kids he spawned. Quill was the only kid who managed to become partially celestial so he thought it would be just him and Quill encompassing all the planets in the galaxy.
shareIt wasn't about destroying everything so much as destroying everything then remaking the universe in Ego's own image-- a way of giving his existence meaning. He initially didn't want to be alone, but Ego is literally so arrogant that he views all other life as beneath him, hence his "disappointment" with other intelligent life. It's all (no pun intended) a big ego trip for the guy, a way of escaping a sense of insignificance, more or less.
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