Why do you care about the phone someone else is using?

I don't mean that in an inflammatory way, I'm just wondering why people always seem to get so worked up about a person's ecosystem allegiance.

I mean personally, I spent a year with a Samsung Galaxy S variant and moved on to an iPhone last year. My preference is for the iPhone despite being used to terminal commands and flashing roms on the Galaxy S. I'm a really anxious person and I just couldn't get the Galaxy S to last more than a month before getting fatal FC's in GApps. But at the same time, I totally understand what attracts people to Android and why they make the switch or never get an iPhone.

So why do you feel invested in the phone that other people buy? Or the advertisements they make to convince them?

Sidenote: I'm posting this in Apple Core, but it's also meant for Android and Windows phone users, but I figure all three parties end up spending a lot of time here.