why should bride should have a less age than bridegroom?

actually in Indian tradition we belive that if bridegroom has more age than bride than their life will move in smooth manner because the bride always leason their husband and do not try do more argument . she accept as it is as their husband want to say . on the other hand husband is so mature than wife so he also understand better way of wife feeling because he has already passed this age 7-10 years before . always both have them good understanding due to their age difference .