海誓山盟只因太年轻
[hăi shì shān méng zhĭ yīn tài nián qīng]
It speaks to young lovers making vows as solemn as oaths by sea and mountain - promising fidelity. Here though, it comes with the wisdom or regret that it's because they were 'too young', suggesting promises often made too earnestly and naively when inexperienced with life's realities.