Who determines the fuel load?
Disclaimer: this answer applies to airlines (operating under 14 CFR 121) but does not encompass all of commercial aviation. 

The dispatcher that is working your flight plans the routing and determines the minimum fuel required from takeoff to destination to furthest alternate with additional fuel as required by regulation you operate under.  On top of this, contingency fuel is added which can account for enroute diversions, cruising at a different altitude, un-forecast winds, holding, etc.  The total of this fuel is the "release fuel" and you cannot takeoff below this number.  On top of this, extra fuel will be added for taxi time.  

Ultimately, it is the captain's responsibility to decide how much fuel is needed.  For slight increases, airline policy may allow captain's discretion to add fuel.  For larger increases in the fuel order you generally need to call the dispatcher and amend the release -- after convincing the dispatcher you need the fuel.  If you decide you need less fuel (e.g. less contingency fuel) you always need to amend the release so you can legally take off.