negative Traits in game play

I always tend to make fairly good Sims on the whole, with neutral/good traits. Occasionally in Sims 3 I'd make a Sim "neurotic" or "absent-minded", but that was as far as it went. I usually make partners different to each other, but with maybe one trait in common. Obviously by neutral traits I mean the skill-based ones, or stuff like "loves the outdoors".
I suppose what I get from it is that my Sims might start off being similar, but I then see how their lives pan out (eg number of kids, their careers, general life events). I think it also ties in to the fact that I tend to be a "family" Simmer, in that I start off with a married couple and get them to have loads of kids.

You can choose 3 traits in CAS, but can earn more through gameplay (you get a 'bonus' trait when you choose the aspiration, plus another one when you achieve that aspiration, for example). If you raise a kid from birth, they can earn loads more. I played one family under the "long" lifespan, and the daughter managed to achieve all the childhood aspirations, plus she'd had the time to max all the toddler skills...