What is the legal context for a determination of a parent's best interest in a child by a judge in a custody application?

The legal context for such a determination was described by Madam Justice McLachlin (as she then was) in Gordon v. Goertz, 1996 CanLII 191 (SCC), [1996] 2 S.C.R. 27 at paras. 49 and 50: 49 The law can be summarized as follows: 1. The parent applying for a change in the custody or access order must meet the threshold requirement of demonstrating a material change in the circumstances affecting the child. 2. If the threshold is met, the judge on the application must embark on a fresh inquiry into what is in the best interests of the child, having regard to all the relevant circumstances relating to the child’s needs and the ability of the respective parents to satisfy them. 3. This inquiry is based on the findings of the judge who made the previous order and evidence of the new circumstances. 4. The inquiry does not begin with a legal presumption in favour of the custodial parent, although the custodial parent’s views are entitled to great respect. 5. Each case turns on its own unique circumstances. The only issue is the best interest of the child in the particular circumstances of the case. 6. The focus is on the best interests of the child, not the interests and rights of the parents. 7. More particularly the judge should consider, inter alia: (a) the existing custody arrangement and relationship between the child and the custodial parent; (b) the existing access arrangement and the relationship between the child and the access parent; (c) the desirability of maximizing contact between the child and both parents; (d) the views of the child; (e) the custodial parent's reason for moving, only in the exceptional case where it is relevant to that parent’s ability to meet the needs of the child; (f) disruption to the child of a change in custody; (g) disruption to the child consequent on removal from family, schools, and the community he or she has come to know. 50 In the end, the importance of the child remaining with the parent to whose custody it has become accustomed in the new location must be weighed against the continuance of full contact with the child's access parent, its extended family and its community. The ultimate question in every case is this: what is in the best interests of the child in all the circumstances, old as well as new? [Emphasis in original.] Current arrangements