It's always a pleasant surprise to bump into someone you haven't seen in years.

bump 的本义是"碰撞、撞击"，在这里引申为"偶然碰到"。 into 表示"进入...状态"或"进入...情况"。 bump into 是一个常用短语动词，意思是"偶然遇见、碰巧遇到"，强调不期而遇的偶然性。