Have you ever been so happy before that you can't help but feel scared, because you know a "crash" is inevitable?

I think it is possible that being too happy can lead to feeling scared or sad. It is ironic but not impossible. This is because we often view happiness as a goal to achieve and it serves as a motivating factor for one to work towards their target (e.g., better work-life balance, happy family, good performance at work etc). Hence, if one feels that their goals has been achieved (i.e. feeling happy and satisfied with life), that leads the to question of “what’s next?”. That idea itself could be terrifying since it brings about uncertainty. It could also be our mind’s defense mechanism in preparing us for negative outcomes – cautious optimism. Therefore, despite being happy with our current state, we remain aware of the possibility of negative outcomes that can prevent us from being complacent with the current state.