1: It's somewhat belivable
Think about it: Yamato, the lone wolf, and Koushiro, the brain. They get along pretty well, and they both seem to have similar personalities (laid back, somewhat prepared, cautious, etc). They don't fight much, if at all, and they also do a lot of things together (They get their crests at the same time in 01). That shows that they are good friends, and love always starts out as a friendship (unless the title of the story is "Romeo and Juliet").
2: It's not Taishiro
That automatically gives them a point. (I am biased! Yes I am!)
3: They look so damn cute together!
^^ One fangirl's opinion, but they just look so cute together! Just take a look at the pics of them together!
4: It's more belivable than most of the other couples out there involving one of them.
I think the only couple more belivable than them that includes one of them is Taito. I'll have a graph comparing the couple at a later time.
New!
5: Time changes people.
True, in seasons one and two they were not romantically inclined to each other, (thankfully, too. The idea of two little kids going at it is icky) They might have grown closer to each other over the years. We don't know, since the epilogue thingy isn't very clear on marrage. It isn't even clear on Sora and Yamato's relationship. Methinks that Yamato married Sora, had two kids, divorced Sora, then married Koushiro. As for Koushiro, he has a daughter becuase he is married and divorced as well. Either that, or his wife died or something....hey, that's a pretty good idea for a fanfic....
Ok, I only have a few points right now. But they are valid points!