I have the same problem, too. Most of the time for me it's just the game doing that, but it's also a possibility it may be a hack. I haven't found out for sure yet, though.
Since I have the InSimenator installed, I usually spawn the aspiration adjuster, then refresh my wants from there. Sometimes it helps, sometimes it doesn't.
Okay. I forgot to mention they sometimes refresh on their own. (When a sim returns home from work, school, community lot; or when the only wants still available are fufilled) And I looked up some more in another thread similar to this problem. Do you happen to have any "anti-spam wants" hacks in your Downloads folder?
I don't have any "anti-spam wants" hacks. Heck, I haven't downloaded a new hack in ages. I tried using the aspiration adjuster to refresh the wants, but it only refresehed that single want (almost always "Ask Sim on Date") while keeping the other three blank. :\
However, the wants DID refresh themselves after the sim came home from work so I guess I could just do that until I find the cause.