Why is it that women feel they're too good to hold doors open for men as an act of common courtesy?

I always look to see if anyone is coming behind me and hold the door open for them. My son has now picked up on this and will normally hold the door open for other people, he's 9. It's just common courtesy. When someone holds the door open for me, I don't see their gender I just see a person doing a good deed for someone else.