I think what Hugo is trying to do is to combine the immense joy that he feels at the beauty of nature with a paean to romantic love. As he watches the little butterflies flutter from flower to flower, he's instantly reminded of scraps of paper from tattered old love letters scattered by the breeze. Springtime is traditionally associated with love and romance, especially in poetry. In the words of Tennyson, it's a time of year when "a young man's fancy turns lightly to thoughts of love." It's also a time of great natural beauty, when the whole of nature seems to burst into life after its winter slumbers. Hugo combines these two elements of spring in a wonderfully whimsical and inventive conceit: that butterflies originate from the torn scraps of love letters written on soft silk paper.