A lot gets said about how a lot of fans are glory hunters etc and how all non local fans have these "stories" as to why they love United, no story or reason is as important as the other, each one is personal to that person!
For me being from Aberdeen and coming from a family of Celtic Fans I never really supported anyone as a really young child, my Dad played Football to a high level as a Semi Pro so I didnt really notice anything else. It was around 1993 just before I was 10 I started to get more into the Pro game would watch the odd Aberdeen game purely because I didnt like the Old Firm, I then discovered through looking through family effects that my Dad had the opportunity to sign for Manchester United when he was 15, he got homesick and stayed here, I am biased but I think he would have made a career as a Pro if he had the ambition but he is an enigma of a man who just wanted to do his own thing, still does, not saying he would have played for United but at least somewhere lower perhaps!, of course, we all know of stories like this but twinned with the Fergie effect/link with Aberdeen I started to follow United and have done so off and an on for years. I am involved with football myself so that always take priority. I will never forget the days growing up in the 90s watching United, for many 99 in Barcelona is the pinnancle but for some reason I always loved the ending to the 1995/96 season best because it was the start of what was to come in the years after.
I do not profess to be a big fan or a better fan than others, this is just my personal story!