https://www.italki.com/article/312/When-To-Use-%E2%80%9CPor%E2%80%9D-And-%E2%80%9CPara%E2%80%9D-In-Spanish:-The-Complete-List-Of-Rules

From my perspective as a native speaker, this is how I´d explain the rule: Para implies a goal or finality. Compré ese anillo para ti — I bought that ring for you— which means your action was intended for someone or something else. You are talking about your goal when you acted. Realizaste una acción con una finalidad determinada, estás hablando de tu objetivo al hacer algo. Por implies cause. One thing or person originated a reaction in you. Compré ese anillo por ti —I bough that ring because of you— which means you are the reason why I bought it, maybe the person convinced you to act that way. When the sentece is about the reason why you do something, you use por. Una cosa es la causa de la siguiente. Si compraste ese anillo es porque la otra persona te convenció de que lo hicieras.