Electricians please, why is my one dimmer hot?

You could have too much bulb wattage for the dimmer. All switches including dimmers are rated for a certain number of amps. If you have say a 1000 watts of bulbs that equals nearly 9 amps of current going through the dimmer. If you still have the packaging for th dimmer check the amp rating. The calculation is volts X amps = watts, or watts/volts = amps.

Other problems could be faulty connections in the switch box or the light fixtures that are increasing the resistance in the circuit therefore making the dimmer switch hot also.

Dimmers are rated for how many watts they are designed to handle. If a dimmer has to operate at or close to its full capacity, it will heat up. Some dimmers have metal fins on the front of them to enable proper heat dissipation. You can either reduce the wattage connected to the dimmer to alleviate the problem, or put in a larger-rated dimmer.

www.delairelectrical.com

Do you have anything other than the "normal" light bulbs in your bathroom, like low voltage halogen, other lighting that uses a transformer or ballast in the fixture to reduce the voltage to the bulb, any fluorescent lights, fixture with a heater

Any of those are an "inductive" load and could easily cause a dimmer to overheat

first of all dimmers get hot, that's how they work. butt if you can burn your finger from it then its over loaded buy a dimmer that can handle more watts.