Aly,

Does it have something to do with their Aspirations / Wants and Fears?

I found this info on the web


Question
What are 'Wants' and 'Fears' and how do they affect my game?


Answer
Everyday, your Sims have new Wants and Fears based on the Aspiration chosen when they were created. The list of daily Wants and Fears for each Sim can be accessed by clicking on the Aspiration icon. The icon is located above the Aspiration meter to the right of the selected Sim's portrait in the interface at the bottom left of the screen.

Your Sims start with 4 Wants and 3 Fears everyday. Completing one of your Sim's Wants will earn your Sim Aspiration points. Completing one of their Fears will make your Sim lose Aspiration points. Once a Want or Fear is accomplished, a new one will appear. Accomplishing Wants will raise your Aspiration meter which will allow you to achieve Gold and Platinum moods (which are even better than green!). You can also use the points you earn to purchase Aspiration Rewards. Remember, each Sim earns their own Aspiration points; they are not shared among the household.