You may have noticed your Mormon friend doesn’t like to go to parties where people are drinking alcohol; maybe he or she also does not want to go bar hopping (even if you are at a legal age to drink); perhaps you and your friends like to dance, but your Mormon friend does not feel comfortable going to certain clubs. You may know about and respect his or her decision to not drink, but just because they go to these places with you doesn’t mean they would have to drink, right? So, what’s the big deal?
The reasoning behind this behavior is very simple. Mormons are taught that they can have the gift of the Holy Ghost with them if they are living lives in line with the commandments of God. This means that they can have direct personal revelation with a member of the Godhead is they are living the commandments. Mormons are also taught that the Holy Ghost cannot be in environments where evil can dwell. Clubs, bars, and other environments encourage lascivious, promiscuous, or immoral behavior; thus, the Holy Ghost cannot enter those places.
Keeping yourself free from temptation and overall bad influences is just a good general rule. You can have just as much fun in places which don’t have to have the lights out. Make your own parties which are alcohol and drug free, create environments where people can dance appropriately, encourage moral behavior in your friends rather than licentious behavior.