Indeed, good for Mia for rejecting Sion and choosing Abel instead. All Abel ever needed was someone to believe in him, to give him the confidence that he can surpass his brother Gain and Sion. Mia may not know it yet but this is the beginning of their (her and Abel) loving relationship.
I think that one of the reasons as to why Abel was flirting with all those girls was that he hoped that he would find the ’right girl’ for him, that special girl. It turned out that he didn’t find her, rather she (that being Mia) found him. It’s true that Mia’s reason started out as a selfish one, but it doesn’t detract from the love that they will have for each other.