I think it's both. A relationship needs to be based on mutual respect, communication etc. But if the connection isn't there, then I think it's harder to do the above over a long period of time.
As I said, I don't believe in the notion of one soul mate. But I do believe you can love someone and not have that special connection. I had my first love at 16, even moved in with him so it was real love, not kiddie crush stuff. I def loved him, but I didn't know until I got with DH that we lacked that special something.
edit to add: I don't believe in cosmic stuff. Just that you only find a couple, maybe one person in your lifetime where there is just 'something' totally special above love.