Manufacturers will often set limits on certain aspects of vehicles - how much pressure you should put into your towers, what type of octane gasoline your engine should use, etc. These limits are there for a reason, and ignoring them could have serious consequences.
That's also the case when it comes to your pick-up truck's towing capacity. This isn't some overly-conservative guideline used by the manufacturer. Towing capacity, also known as gross vehicle weight rating, is the upper limit of how much your truck can comfortably tow.
Ignoring this number and loading up more than your truck can handle is playing with fire. Not only can you damage your truck, but you will be hurting your steering power and braking to the point that you could be in serious danger out on the road.
Similarly, trailer hitches have their own weight limits, divided by class. Be sure that you know exactly what type of hitch you will need for what you're hauling. Keep in mind that a heavy-duty hitch does not magically increase your truck's towing capacity.
If you're unsure about how much your truck can handle, you should look up public scales in your area, which will tell you exactly how much the fully loaded truck and trailer weight. You can then check this against your owner's manual to ensure you're not over capacity.