For yet-to-be enlightened being like us, the reality of impermanent or transitory or fleeting nature of the entire universe is unacceptable, undesirable, scary etc.

As long as the notion of "I", "my", "mine" or otherwise known as "self" seems permanently secure, it's all good and merry.

वज्रच्छेदिका प्रज्ञापारमिता सूत्र (Diamond Sutra)