Why does a comparator changes its state when I touch any node at the input?
Noise, and the fact you just added capacitance resistance and inductance to the circuit you are measuring, especially if the input signal is from a high impedance source.

Is it avoidable. That depends on whether making it go away so you can use your measurement device affects the functionality of your circuit and how important it is to allow someone to take that measurement on the active model.

Adding circuitry so you can test the unit actually reduces the reliability of the system, so you need to justify the addition carefully. 

If it is critical to measure it, for example to set  some reference. Then it is prudent to put the signal through a buffer so it has a low impedance source at the measurement point.