The Social Motive is the Sim's need to go out with other Sims and to socialize. A Sim with no friends, even if they aren't particularly outgoing, will become highly depressed and may refuse to look after their own Motives.
 How To Resolve
Sims are social creatures, thus need to go out on the town in a while, so to speak. Negative social actions, such as flirting when it's not appreciated, will have a detrimental effect on the Social Motive. However, positive actions will boost the bar; such as having a fulfilling conversation with another Sim.
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