Actually, he had a choice about the advancement. He could choose not to advance, but he wouldn't be able to fuse with his body and would only be able to exist as a fake rank 9 Dao Initiate like Golden Sun and Time Traveler, who are neither alive nor dead.
As if being a remnant of a Dao Lord wasn't bad enough, there was also the fact that his current state of existence had been exposed to the Dao Lords in the divine constellation.
"I bet they sniffed me out as soon as I appeared," he said with a chuckle.
So he had a choice, but it was between being a half-dead rank 9 Dao Initiate without a body that every Dao Lord would try to kill as soon as possible or becoming a bona fide Dao Lord. He chose the latter, so the grand force of the universe descended on him to help him advance.
During this process, he fused with his body and finally regained connection with the Spark of Power. This was the most important reason he chose to allow the fusion to happen.
