When a couple has been trying to conceive for a long time without luck, they should undergo fertility testing in order to find out whether it is a case of male infertility, female infertility, or both. If it is a male factor, sperm donation might become the only solution in the most severe cases.
Also, in the case of lesbian couples and single females (single mothers by choice or SMC), the absence of a male partner leaves them no alternative but to use donor sperm. In these cases, and especially among heterosexual couples, accepting the fact that the baby will not share his genetic material with that of the intended father becomes hard for some couples.
However, having a child is not only about sharing your DNA with that of your child, but also about bringing him up, and seeing him grow up in a loving environment. Keeping this in mind, the process for finding and choosing a sperm donor may be easier to cope with.