Leaving aside shipping, I was once introduced to the literary device of "complimentary opposites attract." While opposed on certain issues (leading to conflict), there must be similarities and sympathies in other ways (sometimes in ways that the bickering couple may be totally unaware). Also, there must be moments of regret, voiced or unvoiced, when one part of the couple has taken things too far. That's off the top of my head, but there are a half-dozen or so other guidelines involved.
When complimentary opposites are taken into account, I like the "opposites attract" trope. When they aren't taken into account, it leaves me cold. "Similars attract" also works, but if the couple is TOO similar it gets a little... creepy.
