Even the most dreamy of romantic partners is a human being with flaws that can drive you up a wall. While it's typical for infatuation to shield us from someone's most annoying flaws in the very beginning stages of a relationship, eventually the mask falls, and we are faced with the strange ways our partner chews food, or their inability to keep up with the laundry.
Being compatible with someone is heavily reliant on agreeing to tolerate their flaws, because you love everything else about them so much. Or in some cases, learning to love their flaws as well (and ideally this is reciprocated back to you).
It's a lot easier to have grace for potential deal-breakers after you've already fallen deep in love and been through hardships with someone. At that point, you can see just how worth it your partner is, even if they sing off-key loudly for hours on end.