A Date for the Prom

Why is it that girls "need" to have a date for the prom? Is it because it's what's expected of them? Is it due to peer pressure?
Girls spend weeks and weeks stressing over thier date for prom in a time my friend affectionately dubbed "Prom Date Hunting Season." They have this picture in their mind of a perfect night in a beautiful dress with a charming young man.
At my high school's walk-in today I witnessed two exceptions to the "prom date" rule. First I saw two best friends who were either unable to get dates or didn't want them in the first place, walk together, looking as those they were thoroughly enjoying themselves and having no pressure of entertaining a date for the night. In addtion, I saw one boy walk with twice, once with his actual date and the second time with another female friend who was unable to get a date.
Why do we worry and stress over one night? We should care less about what others think about our choice in prom date or lack of prom date and more about what will make us happy.