The reason I support Aberdeen - I was a toddler on holiday in Aberdeen with my family and my dad took me along to see Aberdeen training because he knew one of the reserve players (Alan Lyons) who had invited us along for the day. At the end of it I got to kick my wee football about with (amongst others) Willie Miller, Gordon Strachan, Stuart Kennedy and Alex McLeish. Ever since then, even though I was too young to appreciate who or what they were, my parents encouraged me to support Aberdeen rather than the OF and as I was fascinated by football that's what I did, and it helped my grandfather was a Dons fan too (everyone else in my family supports Celtic).
The reason I love Aberdeen - Other than the usual reason of "they're my team, so that's it", they just become part of your thought process and it inevitably links into your emotional system. You wait all week for them to play and when they win it's brilliant, and we've (mostly) all seen some wonderful players over the years: Miller, McLeish, Leighton, Strachan, McGhee, Simpson, Weir, Theo, McDougall, Black, McKimmie, Grant, Bett, Mason, Nicholas, Gillhaus, Jess, Shearer, Richardson, Booth, Rougvie, Anderson and Hewitt... Players like that made them an easy team to love, but the bigger reason is probably being swept along in the emotion of a game or a season with everyone else who follows the Dons.