Now, how would he reject reality and substitute his own?

Well, my personal theory is that it had a lot to do with Tifa being the first person he met (at the train station). 'Cause there was so much in his mind about wanting to be strong for Tifa (that's why he wanted to join SOLDIER in the first place, remember?), wanting to be strong and capable so that he could protect her and make good on the Promise. And who was strong and capable (in his mind)? Zack.