WHY DO WE ALL SING THE SAME CHRISTMAS CAROLS?

The word carol or carole is a medieval word of French and Anglo-Norman origin, believed to mean a dance song or a circle dance accompanied by singing. Broadly defined, carols express religious joy and is often associated to the Christmas season. 
It is unclear when the first carol was written but it is believed that circa 1350 to 1550.
During the 14th century carols became a popular religious song form. The theme often revolved around a saint, the Christ child or the Virgin Mary, at times blending two languages such as English and Latin.By the 15th century carols were considered an important contribution to English medieval music. Most of the carols we know today were written during this period.
A Christmas carol (also called a noël) is a carol (song or hymn) whose lyrics are on the theme of Christmas or the winter season in general, and which is traditionally sung in the period immediately surrounding the holiday. The word Carol actually means dance or a song of praise and joy! Early Christians took over the pagan solstice celebrations for Christmas and gave people Christian songs to sing instead of pagan ones. The first specifically Christmas hymns for Christians that we know of appear in fourth century Rome. In AD 129, a Roman Bishop said that a song called "Angel's Hymn". Soon after this many composers all over Europe started to write 'Christmas carols'. By the time of the Middles Ages (the 1200s), most people had lost interest in celebrating Christmas altogether.
This was changed by St. Francis of Assisi when, in 1223, he started his Nativity Plays in Italy. The people in the plays sang songs or 'canticles' that told the story during the plays. Sometimes, the choruses of these new carols were in Latin;
The new carols spread to France, Spain, Germany and other European countries. Most Carols from this time and the Elizabethan period are untrue stories, very loosely based on the Christmas story, about the holy family and were seen as entertaining rather than religious songs. They were usually sung in homes rather than in churches!
Christmas is viewed as a special time of year. Christmas carols are sung in celebration of the season. The carols symbolize Christmas, in terms of gathering the family, being joyous, and getting into the spirit of the holiday.