再见时光
[zài jiàn shí guāng]
It reflects on bidding farewell to a period of life, filled with sweet and bitter memories, and looking forward or reminiscing times past. It can imply growing up, losing or reuniting with old friends, nostalgia for childhood, etc. It can also mean that the netizen cherishes every minute and does not want time to slip through their fingers, hence they keep saying hello and goodbye to it, just like what this username suggests.