Because the beauty of "Valentine" is in its simplicity--there are three verses, all musically the same--and a chorus, and it's just one guitar and one bass. You need either two or three guys to pull this off, depending on how many of them can hum and play bass at the same time. And I'd say the beauty of the song is in its simplicity, but I'm not sure that's the whole story.
Beyond that, the chorus has a great harmony line. It's on the same rhythm as the melody, so it can't really be called a countermelody, but it's one of those harmony lines that's so strong you find yourself singing along with the backing vocals instead of the lead.
And I don't really have much else to say. There's no profound musical or emotional reason to like this song so much. It's just that simple.