To some people, the idea of adopting a "stranger" and raising it as their own child just isn't something that they're interested in. They can't imagine that they would feel a family bond with someone who isn't related by blood. Others are less extreme on the subject, but find that the pull to go through the experience of carrying and birthing their own child, or just to have a child that carries their genes, is undeniable.
It's important before you start the process of adoption to think about whether you'll feel sad that your child won't be a biological part of you and your family. You won't go through the journey of pregnancy, labor and delivery. Depending on the type of adoption you choose, you might be present in the delivery room with the birth mother, but it's not the same as doing it yourself. And if the child you choose to adopt is older, you may have also missed out on important milestones in his or her life.
Not being biologically related to you doesn't make an adopted child any less your own; it's just different. Look at it this way -- if you're married or have a partner, you're not biologically related to him or her, but you still formed a bond.
If you decide that you can't deal with the idea of having a child who doesn't share your genes, however, that doesn't say anything bad about you. It's far more responsible to explore these feelings now than wait until you're in the adoption process.