Unlike most of society, Sam and Dean hold familial relationships above all others, and, in particular, the relationship they have with eachother. Whilst Dean and Castiel do have a 'profound bond', Cas understands, and accepts that he will never be the most important person in Dean's life, no matter their relationship. That title will always belong to Sam, and the same is vice versa. Dean still cares for Cas, of course, but if he were forced to pick between him and Sam, he'd pick Sam every time, and Cas was aware of that fact before he even realised he liked Dean. That is the main thing that sets him apart from Dean's past partners, like Cassie and Lisa.
Cas spent so much of their infinite lifespan observing humanity from a distance, and is happy to finally be able to not only observe humans as a peer, but also play a direct, (mostly) positive part in two whole humans' lives!!! He loves them both a lot, and is still learning what love is and how to navigate emotions for the first time, so why not learn from soulmates, who have love so deep and intense that they have been to hell and done unspeakable things for eachother?
The other citizens of Lebanon don't entirely get their relationship - For example, they often see Dean and Castiel holding hands, or on the rare occasion, giving eachother a peck on the lips, but then they also see Sam and Dean. When the brothers are out together, they always seem to stay within a metre of eachother, preferably with at least one body part touching at all times.
