There are several synonyms for the phrase "lose one reason" that can be used to express the same sentiment. One such synonym is "lose one motive," indicating that the driving force behind one's actions or decisions has been removed. Another option is "lose one rationale," which suggests that the underlying logic or justification for a certain course of action is no longer valid. "Lose one incentive" is another synonym that could be used to convey the loss of a motivating factor, while "lose one purpose" indicates a loss of direction or goal. Overall, these synonyms all suggest a sense of aimlessness or uncertainty when one's reason for acting or deciding has been removed.