There is a theory in psychology called contrast theory in which things are only truly understood through experience of their counterpart. What this implies is that we do not know fully understand the experience of something until we have experienced its opposite. Cold, for example, isn’t completely grasped until we have had the experience of warmth. If all we’ve ever experienced is cold, then cold becomes the neutral temperature, and we can only adjust our experience based on that neutral idea. As the months of winter drag by, the speaker of the poem may feel as if they have forgotten just what the warmth of spring is like. Every day is gray and cold, so that simply becomes their existence and their expectations for the day. However, when that first warm day of spring finally arrives, the experience of warmth is so novel and exciting that they are overcome. The transition from spring to summer or autumn into winter is a gradual slide that isn’t noticed much, but the change from winter to spring is punctuated by sudden bursts of color in the landscape where there was nothing but gray and brown for months. The contrast is so stark that it feels like a time of celebration when the weather finally turns for the better.