It's the love-in-spite-of-differences factor. And /I/ like the tragic side of elf/mortal pairings.
Also, Legolas /Aragorn is completely not logical! Legolas/Gimli is at least somewhat founded on the book. (Though of course not as much as Sam/Frodo, that's not a pairing I'd ship, but it's so obvious it's not even subtext any more.) I mean, they ride one horse together... *hinthint* (Sounds harmless? Try it!) And if travelling the world together for some hundred years isn't a good basis for a relationship, I don't know what is.

But hey, if you don't accept a dwarf as competition, what about that fangirl mob outside your window??? ";-)