Something to consider: Adopted children don’t get to choose whether or not they are adopted, or what family they’ll end up in. Adults make these choices for them. Given their lack of choice in what happened, making their own decisions about how to handle their experiences later on matters greatly.
Of course, different adoptees will make different decisions, for all kinds of reasons. But too often, adults try to dictate how they should feel and what they should do with regard to their birth parents. Sometimes it goes something like “You shouldn’t try to find your birth parents; after all, your mom and dad will be so hurt.” Other times it might be “Don’t search for your birth parents, because it might disrupt their lives or that of their families. They chose a closed adoption for a reason.” Or: “You should definitely search for them, because you’ll regret it later if you don’t.” Or: “How can you refuse to meet your birth parents? Don’t you realize how lucky you are that they’ve reached out and you have the opportunity to know them?” None of this, of course, respects the feelings of the person who was adopted.
Right now, there doesn’t seem to be much regard for your daughter’s biological child’s wants or needs—your perspective seems to be all about your daughter’s desire for this relationship. In fact, there’s so little regard for this young woman’s feelings that your daughter, despite knowing that her biological child has consistently said she’s not interested in meeting, reached out to her on Facebook.
As for why someone who was adopted may not want to meet her birth mother, the reasons are as varied as the individuals involved. Some adopted children feel angry or abandoned by the birth parents, especially if there are other siblings who stayed with one or both biological parents, as is the case here. (This may feel like being the “unwanted child.”) Some adoptees don’t have those feelings—they are living a perfectly happy life—but there’s fear of the emotional turmoil such a meeting might bring. It could raise new questions of what might have been; it could reveal information that the adoptee would rather not have known; it could start a relationship that doesn’t work out, resulting in a loss that could be quite painful on top of whatever feelings of loss the adoptee already has.
I’ve also heard from some adoptees who have met their biological parents that they found the experience disappointing. Despite imagining that they’d have a lot in common with their biological parents, upon meeting they felt as though these people were aliens with different interests, worldviews, personalities, and values—leaving them with a sense of emptiness. Some have told me that they would have preferred to maintain whatever fantasy they had of their biological parents rather than be faced with the much starker reality.