This might be a quite disturbing topic for some of you but I think talking about it won't hurt much. While playing for the n-th time a friend of mine told me to go to Nibelheim and visit the basement. After the long cutscene we sat down and had a long talk - she had a lot to say, and what she said made a lot of sense. Thus we talked and came up with several conclusions...
First of all, Cloud and Zack. Not many take notice of them, as Zack seems to influence only the basic storyline.. Well not exactly. Cloud and Zack are quite bound with each other in a strong yet intimate relationship. Take notice - Mideel Cloud's Mako poisoning is not his first case. After the Nibelheim reactor scene when Cloud throws off Sephiroth, Shin-ra decides to mask everything up - Nibelheim lives again, and both Cloud and Zack are taken into experiment. Zack being exposed to Mako, and being an ex-SOLDIER withstands it, and becomes quite powerful.. Cloud however reacts with Jenova Cell's, and togheter with Mako poisoning and what happened at Nibelheim, he is completely devastated both physically and mentally. The only one that took care of him was Zack. He led him by hand, helped him in every way possible, even decided to take care of him afterwards. Mind you - Cloud's childhood was not one of acceptance. Zack was generally the first one that treated him like an equal and took notice of him. Maybe not the first one that took notice of him. Here is where Sephiroth comes in. Cloud came to Midgar with hopes of a better future, of somehow being noticed. But he failed the SOLDIER test, he became a plain Shin-ra soldier. There he was most probably treated like at home - nobody cared for him much. The second reason why he came to Midgar was because of Sephiroth. He was his hero, the illusionary dream he wanted to reach. And Sephiroth did notice him - maybe because he thought that Cloud might become somebody one day, or maybe he was just toying with him. Nevertheless, Cloud was at Sephiroth's every command. Cloud was so deep in love with Sephiroth, that whatever he would tell Cloud, Cloud would do it. The domination was obvious - adding a sexual side of this relationship even more. He opened up to him. And that is the reason why Sephiroth can easily control Cloud afterwards - it's not only the matter of Jenova cells. Cloud then is betrayed - Sephiroth succumbs to his madness, and thus leaves Cloud with no home, no family, no one. He destroyed almost everything that Cloud had -- and then Cloud lost the only thing that was left as a meaning for him - Sephiroth. After the "death" of Sephiroth for Cloud, he is left wounded in the Mako Reactor. Here we come back to the beginning, where Zack takes care of him.
How does it affect later?
Take note - at the beginning of the flashbacks we don't really know who is talking with Sephiroth at Nibelheim. It's most probably Zack -- it's a dialogue of people who have known each other quite throughouly. Cloud's feeling for Zack were very strong - after losing everything, including Sephiroth, he was left alone, and Zack gave him everything and even more. But not for long - Zack's death came soon, on the very eyes of Cloud. The shock was so strong, that Cloud couldn't accept it. And he didn't. He pushed it away from his memory, forgot about it all, lost the most important part of himself. Here is the moment where all the memories, all his life became blended. Both Zack and Cloud became one. This is why Cloud's Second Mako poisoning is so hard - it just sprang all those memories back. He knew, he felt that it was not him that deserved to live, it was Zack. Living a life of two people, Cloud needed to accept the fact of death of the one that cared for him most and that was the thing that would seperate him to the living Cloud and dead Zack. All along the storyline Sephiroth watches Cloud. After the fall in the Church, the roof and flower bed weren't enough to secure such a fall. Jenova cells, now controlled by Sephiroth, known for the power of regeneration in SOLDIER affected Cloud. By Jenova cells, Sephiroth could continue controlling Cloud - knowing him throughouly it was quite an easy task. He keeps luring him in, confusing even more, making him a more useful puppet, a puppet lost in himself and his own fear.
And what helped him out? His friends. Aerith, who showed that you can feel even after being killed by all your hopes - she led him to understanding that. Tifa, who supported him all along the way, and as an anchor to the past, she played the most important role. She also kept the truth from him, and thus unknowingly she confused him even more. Lifestream was the last chance for Cloud to come back and to understand what has happened. But, Cloud does not stay. He leaves them, he needs to know, and understand. He still hasn't healed.. (quotations are from a Japenese AC trailer) "I don't think I can save anyone. My family, my friends, no one". Standing by Zack's grave (the dead flowers and Zack's sword - a grave he only knew about, took care of it, and made it with his bare hands) he says - "I will live till the moment you did. This is what I have decided". Those who didn't notice what happened between Cloud and Zack won't understand how much it means. And it's quite crucial - it's the reason for which Cloud left..
I guess that's all so far.. If you have anything to discuss, please feel free.. Spare any useless comments.. This topic is serious enough. And please forgive any typing mistakes..