Cloud is the archetypal example of the part of us that wants so badly to be liked or respected or recognized in some way. He was a loner who the other kids teased, so he grew up wanting to be everything they couldn't, and he most likely fixated on Sephiroth because the guy was a huge hero, public figure, etc. Everyone looked up to and admired Sephiroth, and Cloud, being insignificant and lonely, thought in the most primitively rational part of his brain that being like Sephiroth would make people and, most important of all, Tifa like and admire him.