Home is where the fan base is.
When your team wins, no one around you cares.
So you spend a lot of time racking your brain (and phone book) for the friends who want to watch the game with you.
When your team is playing the local team, you feel like your gear makes you a walking target.
SBNation
So when you see a stranger wearing your team's gear, you acknowledge them like you would a family member.
MLB