Romantic progression is a central part of StoryProgression, since it eventually leads to marriage, and the creation of the next generation in the form of children.
Romantic interests are generated by the Flirt Manager on a periodic basis by choosing sims which are attracted to one another, and simulating a date between them. You will receive a "Flirt" story when this occurs, informing you of the action.
After a certain amount of time, the romantic couple may then be chosen for a "Partnering" action, that will set the two sims as "Steady", after which the Romance Manager takes over.
Periodically, the Romance Manager will choose a steady couple and score them on the possibly of marriage. If both sims score positively, the couple will be married by the mod.