Why is it so hard for some & so easy for others?

Tara • 02.22.16 ZJLA Baby #2 is on his way!!!!
Is there any scientific study to see why some have difficulty getting pregnant while others have no problem? I don't get it... I wonder what could cause this? The medicines (BC) we take? The environment? 
Someone had posted the other day how their cracked out cousin keeps popping out kids. Isn't that strange though it seems that people on welfare or those that are on drugs have no issue. The really has to be some type of correlation to drugs and fertility. I have been on birth control for at least 10 years, that has to affect your body in someway when trying to get pregnant. 
Even with miscarriages nowadays it seems like everybody has a miscarriage either it was a hush-hush thing back in the day or it didn't happen I don't know but I do know that nearly everyone I've spoke to has had a miscarriage so that makes me wonder what's causing that compared to years ago. 
Just some thoughts... Ramblings. Glow should add a blog feature because there's a lot of random thoughts such as these That go through my mind and I don't want to post it for people I know to read but somewhere on an app like this would be fine but then again now I have you guys to put in your two cents lol