I guess Sim is happy because they are not in realistic world we live in, Sim live in almost perfect world, more or less, even chance to get immortal, easy to get rich, easy to get love, easy to get job and quit job, easy to get highest promation of job so go on, in real life, lot of people don't had that chance at all, even all of them, there is no disability sim in that game! That's why Sim is happy! LOL
In Realistic life, it's hard to get rich or impossible to get rich (for most people), impossible/difficult to get high rank career (limited number this world had offer, everyone can't be boss at same time, ect, but in Sim, offer everyone to chance get highest rank career unlimited! Everyone can be own boss!) Difficult to get love one or impossible for some people (In Sims 4, create spouse and spouse's personal out of thin air!), and impossible to get immortal or occult to get superpower, more or less.
There is "Sad" traits but it's only bring random sadness here and there, more or less.