He did. Zack did what Cloud couldnt by becoming a SOLDIER... and from what I can tell, Zack seemed like a very personable guy, and maybe even befriended Cloud before it all went bad. Zack was the kind of guy Cloud always wanted to be, the kind of guy who would be able to impress Tifa like Cloud never could (or so Cloud believed). So, the combination of his weakened state, Zack's presence, and eventually being found by the woman he was so desperate to prove himself to all served to create this confused alter-ego sort of deal.