Close, inherent bond drops the linker (我媽媽); ordinary possession of a thing keeps it (我的書). The dividing feature is closeness of the relation, not the act of owning.
我的媽媽 → 我媽媽
你的朋友 → 你朋友
我們的學校 → 我們學校
English never drops the possessive (my mom, my school), so learners over-insert 的 on close relations.