Alright that's your opinion but I have to kindly disagree. Here's why:
Sephiroth wants revenge on Cloud because Cloud defeated him. Why is he upset over Cloud beating him? Because it proved that he wasn't the immortal and in fact beatable.
Which is true. However, his bag of marbles had spilled when Cloud gave him the once-over. Sephiroth, at that point, thought he was a superior race and meant to be a god.
This means Sephiroth actually enjoyed the fame and the power that came from his position.
I don't see how that is inferred from the prior statement. Considering the circumstances and that, in Crisis Core, it's pretty clear that he had no desire for being famous. In the Mako Reactor 5 scene, Genesis says that Sephiroth's fame should have been his, and Sephiroth says it's petty, and that Genesis can be the hero.
He was arrogant. He saw himself as far above the average human. He was special. How could he.... the greatest SOLDIER.... get defeated by a mere grunt. He obviously felt humiliated for it because his biggest motivation in ff7 is proving how powerless cloud is on top of proving he is nothing more than a puppet to be controlled.
Yea, but again, he had also flipped his lid and thought he was the Chosen One of the Ancient race, by this point. You have to take that into account.
You heard him. He always saw himself as different from others. At first usually people feel worse because they don't fit in.... but eventually most people tend to see themselves as special because of it. I don't think I need to stress how arrogant that line of thought is.
That means it was his choice to be in SOLDIER.
I can understand this, but when you watch the Nibelheim scene, both the original and CC's retelling, the way Sephiroth acts is as though he is only
then really realizing the gravity of him being different and special. Not to say he didn't realize he was different before, because obviously he did, but that the implications of that never clicked in his mind, if that makes any sense. As though Sephiroth knew in the past that he was different, but he never really concentrated on it enough to give a damn, until then.
I also don't see how that would automatically mean he chose SOLDIER.
Yes he was attached to gast as a child. Perhaps gast had even been his foster father in my theory before being replaced by someone else when he left Shinra.
My problem with what you said about him being raised a in a laboratory comes from the fact that he saw himself as different from others. He obviously has had interaction with other children his age. And I'm sure many of his fellow SOLDIERS prob talked about there childhoods. This means if he were raised in a lab then he would know that he was completely different from them. Where as in 7 he obviously understood he had led a normal but controlled life.
If he was raised in a lab with no one to talk to accept scientists asking him questions and performing tests... I swear I'd think after he got over his initial shyness he'd prob be a happy go lucky guy like zack in CC. Compared to the lab anything would seem like heaven in comparison.
I think I said this in a past post in this thread, but when I think 'raised in a lab/research facility' I don't mean he was confined to a 10x10 room by himself all his life. He could have very well gone to school, maybe some kind of private school or military-esque school, something like that, and still lived in the lab. It probably would have been a setting more like a hospital or care facility, where he had his own room and a regular bed but so that he could still be monitored. Hell, there might have even been other children
there, and they had tutors that stayed there. That one's just wild guessing though.
The final knock against the lab theory is... would he even go crazy knowing the information in nibleheim? Shinra's controlled him his entire life. I don't think finding out they controlled his birth would make any difference. He's already been a prisoner and a slave.
It's highly likely that Shinra lied to him about his origins. He believed Jenova was his birth mother and knew nothing of Lucrecia, for instance.
Also you can't make someone fight if there unwilling to fight. Sephiroth is obviously willing to fight.... and he obviously enjoys fighting. We don't see him ever coping with the make believe brain chip ordering him to fight.
Sephiroth does seem to enjoy fighting, but that still doesn't prove SOLDIER would have been his one and only choice if he'd gotten a choice. For example, I enjoy and am good at cooking, but I don't want to be a chef. I'm sure there's plenty of other professions Sephiroth would have liked, possibly even more than SOLDIER.
And I've posted my theories on why he might have chose to stay in SOLDIER in my last post.