What is the relationship between MOSFET gate voltage and source voltage?
I understand that a Mosfet is a voltage controlled current device -
  but I was a bit surprised that the circuit below didn't yield a higher
  voltage at the Mosfet source pin (load).


Correct but the important thing you are not considering is that the driving control voltage is between gate and source. So, if you want a MOSFET to turn on properly, you need a signifant voltage between gate and source AND NOT gate to 0 volts.

You are using source followers and that will always mean that if you want the source to switch to the 32 volt rail, the gate has to be higher than the source by several volts i.e. 36 volts or above.

Alternatively, move your load so that it is in the drain connection of the MOSFET and connect source to 0 volts. Now that's a different story entirely and this should do what you want.