Could be to the deeper penetration the position affords; he may be bottoming out inside you and can't get that deep in other positions.

Other reasons may include height differences between the two of you. He may be entering you an an angle to your vaginal canal and bumping the top or bottom instead of staying in the middle.

You get used to it and pretty soon ull think its great :)

probably bc you are a wimp

Angle, and maybe because there isnt enough moisture... but I like it like that sometimes

Think of your cervix as your face, would you like someone to headbutt you in the face repeatedly?

you either dont have enough moisture there and/or are not relaxed in that area.