Instructions
Do not allow your Sim to interact with other virtual characters in the game. If friends or family members call and try to get together, do not allow the Sim to answer the phone. If friends come to the door unannounced, do not allow your Sim to answer the door. Long periods of time without other interaction lead to your Sim becoming depressed and will cause a drop in mood.
Remove all "fun" objects from your Sim's virtual home. Delete all computers, plants, wall art, exercise equipment and television sets from the area. Your Sim needs entertainment periodically to maintain a high mood. If your Sim has nothing to do to unwind after a hard day's work, he or she will eventually get the blues and become sad.
Send your Sim to work completely exhausted. Don't allow your Sim to sleep, even when tired. Preventing your Sim from sleeping leads to that character becoming cranky, which can lead to depression. Sending your Sim to work in this condition leads to poor performance and will result in your Sim being fired from his or her job. This will cement the depression and affect the Sim's mood drastically.