In most cases I just let these things happen as they happen. Or as they don't, whichever the case may be. Most of my sims do eventually choose one special partner, but some choose (and keep) more than one, and some never settle down at all in that way. If it's for story purposes I sometimes add a special sim (or a few) for potential partners. I always make them wait to meet each other on their own somewhere in the world, though. I'm not extremely into realism, but the more organically those things happen, the more fun/entertaining they tend to be for me.