Trying for pregnancy

I think it's really crazy how girls in their teens (or girls at anytime, it just seems to happen the most in teens) can get pregnant completely accidentally, get pregnant the one time they're off birth control, the first time they have sex, etc... But there are women that are actively trying to get pregnant, and they've been trying for months, or a year, or years, and they still haven't gotten pregnant, but it's so easy for some girls to get pregnant accidentally.