UPDATE 6/24/19: Now with a “develop love” mod you can use either on its own or alongside the “develop crush” mod. UPDATE 3/10/19: Now this option won’t appear for pets. UPDATE 2/13/19: Now there’s a version that also allows for you to set crushes between children. UPDATE 2/9/19: Fixed so the interaction is not available between sims who are related. To have both the crush option and the love option you must have both “midgethetree_developcrush” (or midgethetree_developcrush_childenabled) and “midgethetree_developlove” otherwise pick the package for the interaction you want. It’s one-sided, so you can make an unrequited crush/love if you want.
Sims have an option to “develop crush/love” on other sims of the same age group (teen-to-teen or adult/elder-to-adult/elder - no, I won’t be making a version for teen/adult), which will set the “crush”/”love” flag and also trigger the crush/love FX (sound effect, floating pink hearts above their head).