Let me answer this with an illustration...
When we put on our new clothes, we would feel uncomfortable of the
feeling, as it may be new and we might not get used to it. So
itchiness may be there for a while, but over time, we do not seem
to feel irritated over it as the sensation of itchiness is
gone.
Our nerve-endings are made in such a way to adapt to certain
continuous / constant stimuli and becomes dampen towards it over
time. If it doesn't work this way, we would constantly 'feel' our
shirts and pants touching us and it would definitely be
uncomfortable.
I may not recall the details, but if you plot a graph where the X
axis is duration and the Y axis is sensation, you will see that the
line will drop over time.
