Part of this is because it feels good to support people I know, people with a face. Real people.
That thought bugs me. Are the people who work at WalMart not real? Am I not a real person because I work for a large company? Teenage Bank will really collapse without customers, just as surely as any sole-proprietorship would. Are Costco's stockholders not real people? Are the people who work in India or China or any other country manufacturing goods not real people? Are they not worthy of jobs and support? Why is that a person who runs her own business making her own goods is "real", but when thousands of people work together to form a corporation, they become fake?
I know some of the arguments here. Some people feel that the profits a large corporation makes go to people who are wealthy already, who don't need the money. (And yet without those corporations, where would all those employees work?) Wages in developing nations are poor and conditions are bad, and perhaps we shouldn't support companies who treat their workers in such a fashion. (But will wages magically rise and treatment improve when there's even less demand for those employees?)
I don't plan to change my spending habits; there's a quirkiness to products that aren't mass-produced that I also like, after all. But I want to remember that large businesses are also made of people. People with faces. People just as deserving of a livelihood.