Fridge Brilliance: In Complete, Cloud starts to feel sorry for himself as usual, but gets taunted by Sephiroth and is reminded that he is/was a puppet. That's not the kicker. Sephiroth is only able to do this because Cloud has Geostigma which is simply Jenova cells that the tainted Lifestream accidentally gave to a some thousands of people. Jenova cells are said to be controlled by Sephiroth's will. So essentially, when Cloud contracted Geostigma, Sephiroth trolled him at every instance he could. When Cloud is about to have a nightcap with Tifa, Sephiroth is in his head reminding him of how much he sucks. When Cloud shows even a bit of hope, cue Sephiroth giving him a hallucination of the past. Cloud literally had a Kanye West in his head for two years or so.
Sephiroth: "Your Geostigma is gone? That's too bad."
Fridge Brilliance: Some people claim that Cloud proclaiming that there's not a thing he doesn't cherish is a bit Narmish, but it makes sense, since Cloud's finally come to appreciate life, every little bit of it.
Fridge Logic: How did Sephiroth come Back from the Dead, anyway? Maybe I missed something because I didn't see Complete, but it didn't make any sense to me.
It's basically explained in, "On the Way to a Smile - Case of Lifestream: Black." Basically Sephiroth obsessively thought of Cloud, to keep himself alive in the Lifestream and used the three white haired guys as a way to come back, along with Cloud's memories of Sephiroth. Aerith could do the same thing, but she choose not to.
The Remnants were made of Jenova cells, but weren't powerful enough to create a body for Sephiroth to house. Being exposed to a pure Jenova sample, the "Head", it let Kadaj's body act as a new one for Sephiroth's mind to leave the Lifestream.