Whoah, that's like saying that people growing up in different homes are never capable of a loving relationship.
Except that's not what I said, you are the one who for some reason decided to generalize in this odd way.
Alistair was given to templars where he didn't want to be. He knows that he is a bastard and feels that he is an unwanted bastard. Until he got the amulet (or whatever that was, don't remember) he thought the Arl didn't loved him and got rid of him because of it. He thinks joining the wardens is better because he thinks of wardens as something where he can belong. It becomes his surrogate family and yet he is not exactly satisfied with it. His perfect reality in the nightmare is being a part of a family with Goldana (not his girlfriend which is a very telling point). He has a meltdown when he realizes that Goldana is not the person he thought she would be and when the reunion doesn't goes the way he hoped. He copes with everything by making jokes. This particular guy has issues.
Realistically speaking people with issues are not fairy tale romance material, of course when you start comparing Alistair or Cullen to Anders or Zevran, or Fenris, or almost any other male LI in DA their issues might seem tame in comparison to some.
(I played as female noble)