I've been playing around a lot with locking dwarves in rooms together, either to get them to talk to a spouse or friend, or to try to build a relationship between potential spouses. What I've noticed is that they sometimes seem to "Socialize" without actually socializing with someone else. Merely having two dwarves in a tavern zone together "socializing" at the same time doesn't necessarily mean that they're interacting with each other. Indeed, sometimes I'd unlock the door and one would leave, and the other would remain "socializing" with an empty room.
To get them to actually interact, they need to be adjacent at the time they're socializing. Whenever I'd see them socializing while adjacent, that would be correlated with the appropriate happy thoughts (or grouchy thoughts about arguments) or with the formation of acquaintanceships between dwarves that hadn't previously been acquaintances.
So your best bet is to force them to be adjacent by making the room as small as possible (though if you want them to have access to beds, tables, chairs, food, drink, and mugs, you'll still be reliant on luck).
One fix I'm hoping for is for dwarves to intentionally move to be adjacent to one another when socializing. (And, while we're at it, for them to deliberately seek out friends or family when socializing in a tavern.)