I have one particular sim that does not much else but wander off on his own and start fights, even with his best friend. He has the evil + hot headed traits.
My loners or shy sims don't tend to initiate any interactions, though they will respond if another sim approaches them. I also use auto romance mods like Zumi, because I like my sims to just get on with it, rather than me having to direct every second of their lives.
ZeeGee is right, TS3 sims do stay true to their traits. Such as party animal sims being the ones you will find quite often at clubs or bars, or Inappropriate + hot headed picking fights in the park etc.
Apart from the Dislikes Children trait. I've known some of my sims with that trait who quite happily have children, and it isn't really until the babies get to childhood age, that the parent begins to shoo them away.
I mostly like negative traits, I don't like perfect sunshine and rainbow games, but that one makes me feel a little bad for the kiddie sims, so if I notice a sim I like has grown up with it, I will sometimes change it.