The world is so full of things we don't understand and there can be so many reasons :) Even if legit but unaccepted cause it can't be proven :)
I am going through a similar situation. The connection is VERY STRONG and chemistry is great but there are factors that are making me & her rethink and these are IMPORTANT factors (can't and won't discuss them out here).
It's not that I don't go through pain; I DO! but certain things have to be prioritized at some point of time.
And I won't let the decision ever be one sided for sure.
If he loves you, and you dont feel the same way, maybe its best in the long run if he ends the friendship. You cannot know how much it hurts to always be second best, always being around them but never being "with" the person you love. And being reminded of their new love interests but never able to make her happy in the way other guys can.