Can someone explain lovestrucks turn on & turn offs system?

beachfossels  The attractiveness system is more than the turn ons/offs.  It's a combination of turn ons/offs, likes/dislikes, traits and probably a few more things that players can't change/set.  So, depending on what you've set for your sims, it can be hard to figure out why a sim finds another attractive or unattractive.  And you can end up with sims that are either unattractive or attractive to every other sim depending on how you set them.  Basically, if you give all your sims a lot of turn ons and likes, they will be attractive to a lot of other sims.  If you only set a few, it's the reverse.

So, to get your couples to be very attractive to each other, you'll have to tinker with the turn ons/offs and likes/dislikes until you find the right combination for them to be very attractive to each other and not be so attractive to other sims.  It takes time and patience tbh.  Once you get the hang of it though, it gets a lot easier to make sure the sims you want together will be attracted to each other.  

Hope this helps.