但我知道遇见你我开心过
[dàn wŏ zhī dào yù jiàn nĭ wŏ kāi xīn guò]
'But I know that I was happy meeting you' reflects nostalgia over sweet memories, despite possibly current sorrows. The owner of the nickname must have been joyful in past interactions, although present days may lack similar joyous moments.