Must we reason all the time ? Having reasoned thus, here's seven.... Its as many as colors of a rainbow and notes of all the music in the world - I find that intriguing. There are even 7 steps to take together in the ancient hindu marriage rites. Is it but a chance reflection of the more deep relation between colors and music and human emotions ? Why the 7 seas, why the 7 sins, why the 7 wonders ? And 7 days of the week !
Even Lord Voldermort caught on and has 7 horcruxes !!!