Why does an increase in capacitor ESR make the capacitor hotter?
The voltage across the ESR in a capacitor will be very low compared to the voltage across the capacitative part.  As such the current will not vary significantly if the ESR varies. This is true because it wouldn't be a very good capacitor if the resistive component was significant in determining the overall impedance.

This means that it is equivalent to a resistor being fed from a constant current generator.  In that case the power dissipated in the resistor will be proportional to the value of the resistance - P = I^2*R where I is constant.