In his first life, Qi Yang was a scummy bottom, and Mei Chenhuan was a pathetic top. After Qi Yang used Mei Chenhuan up and left him with nothing, he finally fell in love with the fool who was willing to give up his life for him.
In his second life, Qi Yang was reborn, and Mei Chenhuan was reborn too. Mei Chenhuan decided to take revenge on society and become a scummy top, so he tormented the pathetic bottom who threw himself at him every day, body and soul. After he tortured him to death, he realized: holy crap, this pathetic bottom was his Qi Yang!
In his third life, Qi Yang, having experience as a pathetic bottom, decided it was better to be a scummy bottom, while Mei Chenhuan, having experience as a scummy top, decided to be a pathetic top one more time.