I think some people may gravitate towards any certain popular brand (Aeropostale, American Eagle, etc, at least where I'm from) because their friends are wearing a tee-shirt from that brand. I notice this a lot in my school, where one guy/girl says he/she really doesn't think that the tee-shirt he/she's wearing isn't all that great, but if his/her friends are wearing this brand, he/she has to wear it too. The way I see it, I guess, is that sticking to one particular brand is a way of climbing the ladder of social popularity.
On another note, a person may gravitate towards a quality yet unpopular brand to define their individuality. If they see that an unknown, obscure brand is equal in quality to a popular brand, then they might stick with that particular brand, just so that they do not "fit the mold".
Personally, I don't stick with a brand. If it looks cool and is relatively comfortable, then I'll wear it. Though I do tend to prefer a certain brand of shoes, mostly cuz they look cool and they're awesomely comfy.