Your proximity is directly tied to your socio-economic level though. If you're a doctor you work with fellow doctors, administrators, and specialists who are in your economic bracket. You live in a neighborhood full of people who make similar incomes; maybe your neighbor isn't a doctor, but he/she is a lawyer for instance. And as you said, you went to college with people who are now in your same economic bracket. This doesn't mean that a doctor can't run into a grade school teacher, fall in love and start dating her despite making 2-3 times more than her. But on average, the doctor dates and marries a fellow doctor, or lawyer, or engineer, or banker etc.
This is ironically why there's a perception that black men pursue overweight white women. Black men and overweight white women often intersect in terms of socio economic status. They make somewhat similar money, live in similar areas, and thus run into each other.