My perspective:
The Seelie and Unseelie Courts aren't about moral alignments nor political factions, they are expressions of the eternal cycle of life and emotion. The waxing and waning of the Moon, the bloom and the blight, the joy of dawn and the dread of night.
Fey are born from the ephemera of dream, and thus are fickle and capricious creatures, capable of turning from mirth to malice in an instant. Fey have no fixed nature and thus must live by their ideals and their oaths lest they unravel themselves. Nonetheless different fey may find in any of the courts or some will change courts according to the seasons, mood or personal whim.
The Seelie Courts embody growth, vitality, warmth, and delight. They thrive in laughter, the bursting of spring buds, and the passionate revelry of summer nights. The Seelie present as bright, indulgent, and unrelentingly glamorous as they revel in the passions of mortal dreams.
The Unseelie embody stagnation, decay, cold and dread. They thrive in melancholy and ruin, the fall of the leaves, the sting of frost and the dread of darkness. The Unseelie present as austere or terrifying as they revel in the fears that mortals try to deny.
Most importantly though, Fey as creatures of dream and emotion have no understanding of the realities of Mortal life, or its limitations. Thus mortals who seek to trade with the Fey must learn to navigate these intricacies or suffer for their ignorance.