It's not genetic. Not all of my Sims are always beaming from ear to ear and that's kind of normal I guess. I'm like that, I am happy on the inside but keep emotions to myself. Maybe Agnes just isn't very Emotional?
I have never seen why anyone would choose Jamie Jolina or Zelda Mae personally, but if some don't like Agnes but like those two and I like Agnes but not those two then well, it's just a matter of taste.