Please don't get upset. My now 24-year old son is adopted and has "known" since he was about 5. He didn't really understand it then, and it took many years. We just didn't want him to think it was a big deal, so we told him before he started kindergarten.
If you have felt loved by your parents, then you are still loved. If you never did feel loved, then you might not have been because they didn't see you as their own. My point is, nothing has changed. Parents have a difficult time dealing with adoption when it comes to their children. The reason I didn't have a problem was that my son knew I was half adopted (by my dad), and he knew how much I loved him and how I felt about him and my biological father.
People have many different reasons for adopting. Some are good. Some are bad. As I said, if you felt loved, you are still loved, and nothing has changed. When you have children of your own some day, whether you adopt or have them or a mixture, you will understand a bit better. But love is never truly understood - just felt, given, and shared. I hope you have a wonderful life in front of you.