Theme Pairing

Said the Toad to the Kangaroo,
"I can hop and so can you.
If we marry we'll have a child
Who can jump a mountain or hop a mile..."

—Shel Silverstein

A lot of thought usually goes into a fan choosing their One True Pairing...

-snicker-

Sorry, couldn't hold it in long enough. Okay, we admit it. One's OTP rarely has any thought into it going beyond the appearance of the characters involved. If they look cute together, we go for it. Like picking dolls to display on a shelf.

Sometimes, however, a couple is chosen due to their similarities. This is especially true if one's favorite pairing is separated by being in entirely different series, although not all theme pairings need involve such a Crossover Ship.

Examples of themes include:

Similar character tropes (Badass Bookworm, Genki Girl/Keet, etc...).
Similar ages outside the usual Competence Zone (if the series itself doesn't do that already).
Similar powers or abilities (Elemental Powers especially)
Two Ensemble Darkhorses or two Satellite Characters paired together.
Identical or similar names.

That's love, baby! Okay, not really. But truthfully, people do get along better when they have something in common, so the idea isn't total bunk.

The basis of many a Crossover Ship.