It is hard to understand why this happens, but there are a lot of factors that can affect fertility. Age, lifestyle, weight, some problems in your reproductive system, low sperm count (for men), hormonal disorders like PCOS, genetic disorders, autoimmune diseases, and sometimes there could be no reason at all (or what they call unexplained infertility).
No matter the cause, women who cannot conceive feel all the same: alone, scared, and lost. It is comforting to know that there are now safe alternatives to have a greater possibility of conception, like thru IVF, and if that is not possible for you, there are still other options like surrogacy and adoption. But, remember there are lots of women (and men) who share the same struggle and experience the same amount of grief. . .you’re not alone and you are certainly understood. Live on and live life.