The Sims 3: Why is my Sim so unhappy?

Supposedly this is due to a bug that causes the well rested moodlet to apply a permanent negative mood effect, instead of a temporary positive one. At least this is according to this fix. I use this fix and it does seem to make the mood work correctly.

To add on to Xantec's answer:

EA managed to totally screw up the "Well Rested" moodlet with the LN patch. For every sleep cycle sims would get a semi-permanent mood impact in the amount of buff value minus 25. So for the cheapest bed, the mood would drop by 15 per day. The "Well Rested" moodlet is only the most obvious case by the way, but not the only one. This mod will fix that issue for you.

So it looks like your Sim just has a really, really, depressing bed. Either buy a better bed, or download the mod Xantec linked to to fix it.

I have had this problem before, and I fixed it by saving, quitting, reloading and then making my sim do something that would usually make him happy. It worked for me, but obviously this seems like some sort of bug so YMMV. Good Luck!

I had the same problem, I can see in the picture that your sim is not fullfilled (mood effect box). I believe that's the problem.

If your sim hasn't been fullfilled lately, they seem to get depressed with out it showing up in the mood effects box.

Try to fullfill your sim as much as possible and you probably won't have the problem again.