I think the underlining problem is that dwarves lack any social initiative. And as far as I'm aware, there isn't any system in place to accommodate this, so nothing is technically bugged up.
In real life, you generally find people you enjoy interacting with, and you seek out opportunities to do so. In DF, dwarves will just randomly socialize with whoever happens to be next to them in the tavern/temple/library/whatever. Two dwarves that could have been friends just don't because they're standing on opposite ends of the tavern, instead talking to people that'll be a social "deadend" forever. The chances that they'll both be in the tavern at the same time, and happen to stand next to one another is so slim in a normally functioning fort that it hardly happens.
There's also the issue that very, very few activities actually result in the dwarves talking to one another and building up a relationship. You'd think two miners that have worked side by side for three years would have some relationship with one another, but unless they've hung out in the tavern and stars aligned for their meeting - nope, nothing will happen.
I think this is also why lovers that could marry take so long to actually do so. Again, these dwarves won't go out of their way to interact. So eventually, after randomly bumping each other over the course of a few years, they'll finally accumulate the social points or whatever to marry.
The best system, in my opinion, is one where dwarves would still randomly socialize, but would devote special time to socialize with dwarves that they actively enjoy interacting with, so passing acquaintances that have the potential to be friendships actually become so. A similar system could be used for lovers.
The starting seven obviously cheat this lack of initiative by forcing some abstract socialization for their relationships upon embark based on their personalities.
The bedrock for socializing and relationships are in place, but it actually needs the finish to work.
edit: it seems that Patriklundell basically iterated my exact thoughts a week ago, whoops. But as Toady said in his fotf reply, there are no short term plans to add more systems. So for now, relationships are just going to be half-baked.