TTC vs. not TTC

i've never understood why or how the people who try, who deliberately have sex to conceive, who keep their bodies healthy, somehow have the worst luck getting pregnant. 
The people who take the necessary precautions somehow get pregnant, when the ones trying to conceive take years to have a baby.. 
I don't understand why people ttc have such horrible luck having a baby.. 
it makes me really sad. :(