Why are some women unable to produce breast milk?

I really wanna give my daughter strictly breast milk in the first year of life, but I've heard stories of women being unable to produce enough breast milk. My boyfriend was telling me about how our friend was unable to produce enough breast milk for her son and she had to give him formula right off the bat. Is there any specific reasons why this happens? Can it be avoided or is it totally unavoidable & it just happens sometimes? I eat very healthy, I'm gaining enough weight, and I exercise at a reasonable level. I'm really worried what if I can't produce milk for my baby?
 
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