because in Chinese culture, dragons are associated with the emperor and phoenixes with the empress. Together, they form a symbol that represents everlasting love between two opposites. This symbol also relates to yin and yang.
Not only is Kris’s symbol a dragon and Chanyeol’s symbol a phoenix but they are also opposite in nature, like a yin-yang: Kris is serious and Chanyeol is always upbeat. They complete each other.