I think that for us to understand why Sune is Chaotic is to look to her inspiration, Aphrodite.

Aphrodite (specifically Aphrodite Pandemos) represents the more chaotic force of nature that is love. Love as many Western philosophies believe, isn't something that can be predictable or contained, it is a curse, a sudden infliction, and it is chaotic. It leads to chaotic events (see all the sit-coms, rom-coms, and love tragedies).

Love in itself can be more predictable and gentler aspects such as kindness, nurturing, the love of family and a life-long partner. But Aphrodite Pandemos, and to a degree Sune, are goddesses of the types of love that the Greek would call Eros, Ludus, and Mania as well (in addition to other aspects of love). This is love that is passionate and all-consuming, it is the game and the chase, and it is the obsession and desire.

Add then in a dogma that "love conquers all", it also bends to D&D's philosophies of opposing society and laws. Sune believes that if two people love eachother, then laws and social norms shouldn't stand in their way of happiness! So if they must break laws and break against social norms to be together, then they does it with her blessing, because "love conquers all".

Setting love (and beauty) above all else will lead to clashing with rules, laws, norms, and expectations.

This is why Sune is Chaotic, but she is Good.