No.
My best friend came upon an interesting situation recently. She's fifteen years old, and she found that she has a half-brother who's thirty-something. He lives a state away, and contacted her and her mom. Toni always thought she was an only child, so this came as a shock, but she was excited to meet her older-half-brother.
She said that he wasn't bitter at all, and her mom started crying when she saw him. They're friends now and keep in touch.
It's ironic because Toni's mom had her brother while she was in her twenties. She was going to get married, but once she found she was pregnant her boyfriend bailed. So she put her baby up for adoption, being she's 110% Pro-Life. Later, when she was married and READY to have a baby, she couldn't get pregnanty with Toni. It took years and years to get pregnant, and I believe she was in her early fourties, which isn't something you see quite often. It's ironic because when she wasn't ready, she got pregnant, but when she wanted a baby more than anything, it was very difficult for her to get pregnant.
To make a long story short, sure there are adopted children out there who are bitter, but there are many out there who realize how much their parents must have loved them to give them up in the hopes of a better life for the child.