It makes sense that there is correlation between attachment style and well being. After all the relationship a child has with it's mother is the first relationship he or she develops. If this relationship is a good one, the child has a basis for all of his or her other relationships. If it isn't good or secure, the child doesn't have a good idea of what a good relationship is. Another factor that might play into well being is how the child is exposed to social situations. The child might have a secure attachment, but if he or she isn't exposed to social situations with other kids then he or she might have a harder time making friends later in life. He or she might not understand things like sharing or how to act around other kids.
This is a good point!! It definitely supports the importance of nursery schools, especially for only children. Without even a sibling to play with a child may not learn the concept of sharing and could have trouble with this when he finally enters elementary school.
