No one in our family are into footy apart from me and my bro. So when me and him were kids (6 and 8-ish) we liked footy but didn't have anyone to support. Then my brother's bestest friend, whose family had moved up from Southampton, asked if he'd support them with him. Like the d!ckhead he is he said yes. Then my brother asked me the same thing, and like the d!ckhead I am I also said yes. That was ~30 years ago and it stuck.
Incidentally, when I was 16 or so I had a friend without a team and managed to persuade him to support Saints by pointing out that we had Shearer, the best striker in the country. Then we sold him to Blackburn and he switched allegiance, the b*stard! He's now a de-facto Arsenal "fan" due to his brother in law.