What makes people float?

I guess you mean float on water when you are swimming. I think the main reason is that your lungs always contain a sizable amount of air. Even if you breathe out as hard as you can, your lungs will still contain almost three pints of air. And if you take a deep breath, they will contain much more. So your body is a little like a bottle full of air.

People are not all alike in their lung capacities, and I suppose that’s why some people seem to float more easily than others.