I think many people choose being homeless over giving into the pressures of our society like having to sustain a job in order to pay rent and go out for dinner etc... and all that stuff that 'normal' people desire. I've met homeless people that split their time between SF and some hills near Santa Cruz and only come into the city to sell some hash or hang out ith friends for a couple of days... but yes there are also many people on the streets who don't want to be there too.