The rabbis taught us to have the lights left on throughout the Shabbos. The lights that are lit for the holy Shabbos, are symbols of the harmony that must exist between husband and wife. On a simple level, we understand how a dark house can cause accidents. Accidents lead people to nervousness and anger. The prohibition of having a fire in the home on Shabbos, can be understood figuratively.
Fire represents anger.
The Torah is warning us to avoid anger on Shabbos. If there have been any tension in relationships over the week, before Shabbos is the time to resolve it. One of the great Chassidic rebbes was accustomed to asking forgiveness from his helpers around the house. When Shabbos entered, there was no trace of anger in his home. Shabbos is when the family spends a lot of time together.