Well-being could be associated with the extent to which a child has developed interpersonal skills. A child with a secure attachment is more likely to develop better social skill from increased interaction with his caregiver and thus be more be better off later in life. Thus, social skills would represent a confounding variable that affects both the IV, attachment type, and the DV, well-being later in life.
I agree with your post because I think social skills have a great deal to do with how happy people are and therefore their well-being. Being able to be social with friends and family is important to many people and thus, can enhance one's well-being.