Under California Welfare and Institutions Code section 361.5, subdivision (c), the juvenile court "shall not order reunification for a parent" described in, inter alia, section 361.5, subdivision (b)(10) "unless the court finds, by clear and convincing evidence, that reunification is in the best interest of the child." ( 361.5, subd. (c); In re Ethan N. (2004) 122 Cal.App.4th 55, 64 "Once the juvenile court finds that one or more of these subparts of section 361.5, subdivision (b) applies, the court is prohibited from ordering reunification services unless the court finds, by clear and convincing evidence, that reunification is in the best interest of the child".) "The burden is on the parent to change that assumption and show that reunification would serve the best interests of the child." (In re William B. (2008) 163 Cal.App.4th 1220 at p. 1227.) Serving the best interests of the child is the fundamental goal of the juvenile dependency system, and underlies the three primary goals of child safety, family preservation, and timely permanency and stability. (Seiser & Kumli, California Juvenile Courts (2008) 2.11, p. 2-22.) "The concept of a child's best interest 'is an elusive guideline that belies rigid definition. Its purpose is to maximize a child's opportunity to develop into a stable, well-adjusted adult.' " (In re Ethan N., supra, 122 Cal.App.4th at p. 66.)