Project 03 - Love-o-Meter - Why no resistor for temperature sensor?

I am trying to understand why the temperature sensor is not in need of any resistor. The book does not really mention it. I understand why the LEDs require a resistor to lower the Amps to a level they can handle (20mA to be exact).

But why does the temperature sensor not require any resistor? And where would I find this information in the component's data sheet?