Miis tend to fall in love by chance. You can help your Mii confess…or not. You can choose the reason for not confessing. However, if the romance isn’t pursued, then there goes for a depressed Mii. When you decide to help confess, you can choose the confession style and location (which depends on what you may have unlocked). I would probably say the worse case scenario is when the Beloved Mii does not even show up. Hopefully our Miis won’t have to go through something like that.
Now if the Beloved Mii is already in a relationship, there’s still a chance. Most likely though, the significant other will interfere with the confession. I can’t tell you how many times my Mii would ask someone out and then all of a sudden, another Mii butts in and is in love too.
Surprisingly, if the Mii that interferes in is reject, s/he doesn’t get depressed, but if the asking Mii is rejected, s/he does get depressed.
Round 2 With Luigi:
Luckily, if the Mii is accepted to become a sweetheart, they can work on getting married and eventually have a baby. I believe that the odds of being accepted or rejected in a romantic relationship depends on their current relationship with each other–whether they’re acquaintances or good friends (which you can view by pressing the L or R button in the Mii’s apartment). Something else to notice is when a Mii askes how another Mii might feel about them, whether you say if they’re a great match or if they’re not compatible, will affect the way s/he feels about other people.
Now if you’re trying to prevent Miis from falling in love, a very useful trick is to set them as “Other Relative” in the Edit Mii menu.
If you save often, you might be able sort of catch your mii before s/he falls in love with the wrong person by resetting it before the prompt comes up. But then that’d be playing with fate. Maybe.