mixture of the above for me...
I have a brilliant relationships with David, in our nearly year of being together, we havent had a single fight, we went through a bit of a rough patch for about two weeks, but that has completely gone, and it was only due to lack of communication (gee, being in nz wouldnt cause that would it!?!?
I remember my mum insisting that communication is the key to any good relationship (which would be why her and my dad are always saying I love you to eachother and to us kids too) so I put that, as I agree.
Trust is also a big one but not as much as communication. Friendship, I think, is better than love though.