To me, it's that Sam/Dean really makes *sense* in the Supernatural universe. They are about as far away from 'normal' as it's possible to be without having superpowers, and right now *all they have* is each other. They are not going to be able to find a nice girl or boy to settle down with that will be able to understand them or what they do. And the relationship between them is canonically complicated enough that it doesn't seem like a huge leap to me to add in a bit of lust to that equation.

Also, hot *damn* do I see the UST. I see the eyefucks, I see Dean slapping Sam's ass *in canon,* I hear the way that Dean says 'Sammy!' when he thinks Sam's going to die, and my brain goes 'they're *so* fucking.'

Hope this helps!