This quote by Rumi is a profound reflection on the nature of life and the human perception of reality. It suggests that our deepest regrets aren’t about leaving this world, but rather about the intensity with which we believed in its reality.

The first part of the quote, "No one looks back and regrets leaving this world" implies a perspective of life after death. It suggests that once we pass away and perhaps gain a broader perspective, we don’t regret the act of dying or leaving the physical world behind. This could be understood as a spiritual or metaphysical perspective where death is seen not as an end, but as a transition to another state of being.

The second part of the quote, "What’s regretted is how real we thought it was" delves into the idea of illusion or ‘Maya’ as it’s known in Eastern philosophies. It implies that our worldly life, with all its pursuits, struggles, joys and sorrows, is not as real or as significant as we believe it to be. We tend to get so engrossed in our daily lives, our relationships, our jobs, our ambitions, that we lose sight of the bigger picture, of what might truly matter in the grand scheme of things.

Applying this to our modern world, we often find ourselves caught up in material pursuits, social media, and constant busyness. We are often so engrossed in these ‘realities’ that we forget to question their true significance, their true ‘reality’. This quote can thus serve as a reminder to step back, to question our assumptions about what’s important, and to not lose ourselves in the hustle and bustle of daily life.

In terms of personal development, this quote could inspire us to seek a balance between engaging with the world and maintaining a sense of detachment. It encourages us to participate in life fully, yet not get so entangled that we lose our perspective. It could also inspire us to explore deeper questions about the nature of reality, the purpose of life, and our place in the universe. This could lead to a richer, more nuanced understanding of ourselves and our lives, and ultimately, to a more fulfilling existence.