I think the critical aspect here is the management system. Pair programming makes total sense when tasks are owned by the team. In that case it's in everyone's

I think the critical aspect here is the management system. Pair programming makes total sense when tasks are owned by the team. In that case it's in everyone's interest to improve the team's overall velocity. That means time spent mentoring juniors or working together on tasks is well spent. When tasks are owned by individuals, it's in everyone's interest to increase their own velocity which means maximizing time spent working on their own tasks. People are disincentivized from helping other people complete their work or mentoring them, because that directly reduces your personal velocity.