What causes an engine to run hotter than normal while towing a trailer that is within the vehicle towing limits?

What causes an engine to run hotter than normal while towing a trailer that is within the vehicle towing limits?
Just to put things into perspective here, let's say you are about 150 lbs. and you are able to pull a wagon weighing up to 75 lbs., You know you can pull the 75 lbs. you have done it 100 times before, but you are still going to feel your body temp. rise and possibley even sweat due to the fact that you're pulling that extra 75 lbs. rather than walking your average pace. The human body wasn't made to pull things at a constant daily rate. The same holds true for your truck. It is made to handle the load when needed, but you are putting more wear and tear on the engine causing the temp to rise slightly because it is having to work harder than it was originally engineered for. If the truck were meant for constant hauling, it would have a Caterpillar engine or something engineered to handle a heavier load on a daily basis. But depending on the truck you are towing wiht, the engine may not be rated for the same weight as the suspension under it. Also the type of fuel your are burning makes a big difference, if it is gasoline, it will burn hotter regardless, but if it's diesel, they are made to handle slightly higher tempuratures.