I generally dislike 1x1's. Even with the best of partners, you begin to understand characters so intimately you can predict almost every single thing that will happen. At a point, to me, it feels like you lose the element... of interaction. Even if you have a second person brainstorming ideas and plotlines, fact is, at some point your characters will be fleshed out to the point that other person might as well be an extension of yourself and vice versa.

I staunchly believe that the fundamental and guiding principle of roleplay is interaction. If you can start predicting the outcomes of interaction with even just a 75% accuracy, it defeats the purpose of giving control to someone else, especially when you get up to the points where you're working on these literary tapestries instead of just weaving a few things together and hoping it doesn't fall apart. 

Adding a third person changes the dynamic greatly. It only goes exponentially further from there. Increasing interaction within a roleplay strengthens it down to its very infrastructure. It's true that weak interaction can be detrimental, but so can an abrupt lack of it. This definitely relates to the point being made about forced romances, or just their likeliness. It's a strong plot element and really it's an aspect of human nature, too. They appear to be convenient, strong interaction between characters when they an easily be the exact opposite. 

However, this does revert back to a previous point. Why 1x1's tend to bring forth some of the best roleplays between people, and the answer to that is simple. With just two people, if they're compatible writers, it's really easy to create a thriving roleplay and work off the low levels of interaction by manipulating the world around them for the characters. The plots can be so character-centric, the events full of character development and bonding. Essentially, it's easier to engage in more difficult aspects of roleplay because there are fewer factors. It is this relationship, this ratio, that makes it easy to respond to interaction in a healthy manner because there truly is less of it. At the same time, it's harder to keep a plot from running stale or a dynamic from getting old if there is such limited levels of interaction and factors being incorporated into a plot and/or setting.