There are many reasons why people choose to adopt a child. Although the most basic reason is a desire to build or expand a family, the specific reasons that motivate each adoption vary.
Some adoptive parents choose to adopt a child because they are infertile (medically unable to bear children). One or both partners in an adoptive family may be infertile. The most common reason a female may be infertile is ovulation issues. This may include conditions such as Polycystic Ovary Syndrome (PCOS). Unlike male fertility, which tends to stay viable into old age, female fertility begins to decline sharply at or around a woman's 35th birthday, and by age 45 or so, it may be more or less impossible for a woman to conceive naturally. Given the large number of women who have chosen to put off having a family until they have established careers, this problem has unfortunately become increasingly common.
Infertile couples that seek to adopt may have no children or they may have existing children that they conceived when they were younger. In the former case, the adoptive parents may have attempted for years to conceive a child, but ultimately be forced to the conclusion that they cannot have their own natural child due to infertility. They still desire to raise children, and make a choice to adopt. In the latter case, the adoptive parents may wish to have a larger family than they currently do and be unable to accomplish this naturally due to infertility. In addition, the adoptive parents may specifically wish to add a girl or a boy to their family.
Infertility is not the only motivation for adoption. Some adoptive parents may have learned that while they can have a natural child, they are at risk for passing on serious genetic or medical conditions and so choose not to attempt a natural pregnancy. Alternatively, a potential birth mother may not be able to risk a natural pregnancy due to her own health complications and choose adoption over the risk of pregnancy.
Some families choose to adopt because they believe they will be saving a child who otherwise would not grow up with the benefits of a loving and supportive family. Such a belief in the goodness of saving a child through adoption often has its origins in the adoptive parent's religious, ethical, and/or emotional feelings and their desire to make the world a better place, even if only for a single child or a few children. In this case, adoption is a means of saving the world, one child at a time.
Still other adoptive parents choose adoption because they lack an appropriate partner. It takes genetic material from both a man and a woman in order to produce a child. Single people and established homosexual couples may choose to adopt a child rather than use a genetic donor(s) or surrogate to produce a natural child.
Finally, a very common form of adoption in the United States and other Western Countries is stepparent adoption. In this case, one or more partners from a couple that have married and combined children from prior relationships into one household choose to adopt the birth child or children of their partner.