This would be a cool feature to toggle on/off. I feel like most of the time I like to choose which sims to pair together, but for the sims that I don't really know what to do with, it could be fun to see how they feel towards other sims naturally. Or even if I already have a plan, out of curiosity I could see who my sim likes and maybe I'd change my mind out of interest or let them have another partner to spice up the story.
What I miss about playing TS3 is the ability to set their astrological sign, fave food...etc. Maybe adding that can play a role in the whole vibe/chemistry/attraction thing.