Scholars explain that being "united with him" uses a powerful agricultural metaphor, like being "planted together" or "grafted in." This isn't a loose association but a deep, organic union. Just as a graft shares the life of the tree it's joined to, believers are so intimately connected to Christ that His death becomes their death to sin, and His resurrection becomes their source of new life. This union, as commentators note, is a divine act, not our own achievement.