understanding the graphs of tau vs R in an RC circuit, and tau vs 1/r in an RL circuit
Think of the time constant of a circuit as the amount of time it takes for all the transient stuff to die off.

Now, look at the two circuits in the steady state: 
   For the cap circuit: The voltage across the capacitor is V and the current through the capacitor is zero.
   For the inductor circuit: The current through the inductor is V/R and the voltage across the inductor is zero.

For the capacitor circuit, doubling the resistor will make the charging current half, and so it will take double the time to reach steady state. So, the time constant is proportional to R. 
For the inductor circuit doubling the resistor will make the final current half as much, as so it will take half the time to reach steady state. . So, the time constant is proportional to 1/R.