Why is it easier for some women to get pregnant than others?

I’m ready to be a mommy and it’s not happening for me…I have a friend who has gotten pregnant twice her first try and had an abortion both times (which is her right!) but with me I’ve been trying for 6 months with my husband and nothing, just period delays but no baby, I’ve gone through so many test and nothing is wrong with me from what I’ve been told, I keep track of when I’m going to ovulate etc etc but nothing and now my brother in law’s wife is pregnant and it’s just hard for me to be happy when I feel like I’m failing in that area😔