Thought bubbles appear above a Sim in the following situations: sleeping, talking, and when a motive is low and they are desperate. Each symbol shown has a meaning. When Sims are conversing, the thought bubble represents what they are talking about. While most of the time, it may seem the thought bubble is not interesting, it is useful in some situations.
A Red Bubble
When the thought bubble picture is shown in red it is time for you to pay attention. They are trying to tell you a motive is extremely low. For example, if you see a red bubble with a basketball or TV, it means the Sim needs to have fun. If there is a picture of people kissing, a Sim needs to socialize with other Sims.
Thought bubbles can be interesting if you keep an eye on them. While they are sleeping you can sometimes see pictures of another Sim they care deeply about. Thought bubbles are just a small part of the game, and gives Sims a way to communicate to the player.