An active circuit generates heat … sometimes damaging heat.
We all know this … The power dissipated in an element is equal to current squared times the resistance of the element.
The current in a switching circuit is equal to:
As clock speeds increase, dt decreases, current increases … heat increases.
Thermal resistance can be considered an analogy to electrical resistance – thermal resistance opposes the flow of heat as electrical resistance opposes the flow of current. A voltage proportional to both current and resistance is developed across an electrical resistance as a thermal potential – 
T – proportional to both heat flow and thermal resistance is developed across a thermal resistance.