Why is that the pleasent feelings last for small time while unplesent feelings seem to last forever?

Why is it that when we feel pleasant and positive feelings, the state of heart-mind is evanescent while when we are feeling down and unpleasant feelings that state of heart-mind feels like everlasting? The default state seems unpleasant then we practice mindfulness and meditation, the pleasant state is attained for some time, as soon as it comes the mind tries to grab hold of it, it's gone. My question is about why the time for which they last the way it is?