Everybody is familiar with this phenomenon. Most of us have a friend or relative who can clear out a room when they kick off their shoes. And even the sweetest smelling person can do a decent job stinking up a pair of shoes by running a few miles in them. So what's going on here? Why do your feet have a stronger odor than the rest of you does?
The main thing that feeds foot smell is sweat. With more than 250,000 sweat glands each, your feet are among the most perspiring parts of the body. In one day, each foot can produce more than a pint of sweat! Sweat is basically just salt and water, though, so it doesn't have a distinctive smell of its own. The smell is actually caused by bacteria on our skin that eats the sweat and excretes waste that has a strong odor. It's perfectly normal to have bacteria on your skin, and it doesn't ordinarily produce a noticeable smell, but sweat attracts bacteria and gives them a whole lot to feed on.
Of course we sweat all over -- our hands have a comparable number of sweat glands, for example -- and most of the rest of our body doesn't particularly stink (the armpits being a notable exception. So what's different about our feet? The answer is our socks and shoes. The sweat our feet excrete can't easily escape into the air like the sweat our hands excretes -- it all collects on our skin and in our socks. The bacteria love this dark, damp feast and have a sort of feeding frenzy. When you take off your shoes, the smell that hits you is all the bacteria excretion that's collected on your feet and in your socks and shoes.
The main reason some people's feet (or more precisely, some people's socks and shoes) smell worse than other people's is that some people sweat more than other people. This is just one of the many variable physiological qualities of human beings. This is also why sometimes your feet smell much worse than at other times -- it all has to do with how much you sweat.