Amps get hot because they arent 100% efficient (meaning, electric power (DC) will be converted to electric power (AC) + heat).
By changing your omhms your resistance will go up (switching from 2 to 4 ohm) or down (from 4 to 2 ohm). Less resistance means more power that can by put out (as you can see in the amp specsheet). More power means more heat.
Inside your amp there is a part that changes the amp's internal voltage up (sometimes as much as 100V). That part generates heat and power. If you have a higher battery voltage or a lower ohms rating (because then you dont need as much power) the voltage up chips doesnt have to work as hard, meaning less heat.
Parts which doesnt have to work as hard means a longer lifespan. If a chip runs on a higher temperature for a long time it will wear off faster than a chip on a lower temperature.