I think I can relate to this very well. I've been wondering about this about only 1 person I know, because what has happened (and keeps happening) is different than anything else.
To give examples: we have freakishly a lot of things in common, like tastes (narrowed down to picking the same songs, movies, books, foods, clothes you name it) many more times than I can remember. We've known each other for almost 3 years so it hasn't been educated in time, it's just always been there. Plus we're both very head-strong people, very unlikely to get easily influenced :)
We're keeping in touch on an almost daily basis, mostly on social media (by phone very rarely), because we're both very busy people.
What's really been weirding me out for a while now is that I can feel so strongly when he misses me or has been thinking about me a lot, to the point of waking up at 1 am or so, full of energy and on specific purpose to check my FB account for messages. And every time this has happened (4 or 5 times) he had also been awake and had needed to talk to me (nothing serious, just feeling lonely or had missed me).
This is precisely why I can't help but set him aside in my mind as special from all of my other friends and I wish I wouldn't do that but I keep getting reasons to keep doing it.
No, it's not because he's a boy & I'm a girl, I have plenty of other guy friends (some very attractive) that I don't feel like this about and we're both in long-term relationships with other people.
I have also had quite a lot of very vivid dreams about him - and I have a very hard time remembering what I dream, but these I can almost see in my mind even now.
The dreams where about very nice things like us dancing together, taking car/motorcycle rides together or just talking and being silly together. What is common to all of them is a vibe of mollifying , pervading warmth (similar to what one gets after a really good massage) that I also got from him - but not as intensely - on the few occasions when we spent time together in person. I can remember clearly how those occasions left us both so charged with energy that we couldn't sleep for the following night and I wonder why because our communication in person is not so efficient as in writing - we're both quite shy and introverted :)
I've tried setting some space between us, so as to stop giving him special treatment compared to my other friends, but I always ended either feeling very depressed / empty after a few days or he contacted me to ask me where have I been, is everything ok. It's almost like a very hard addiction.
And we've been doing this for 2 years 1/2 and we can't seem to get bored of talking to eachother.
Now I'm again on one of those nights when after sleeping for 4 hours or so (not nearly enough for me to feel rested) I suddenly woke up because I had to check my FB and found he'd wrote to me to ask where've I been for the last couple of days - too busy with work for FB...
I'm not one to believe in neverending love, soulmates & chick-flick cliches of the sort that's why I don't want to risk ruining our friendship for any of that. I just wanted to share my experience here and hopefully we'll have some more input from someone more knowledgeable in these kind of things :)
nest of luck to you too! ;)