曾经只因曾经而念曾经
[céng jīng zhĭ yīn céng jīng ér niàn céng jīng]
'Thinking of the Past Only Because of the Past' conveys nostalgia deeply, where 'the past (once)' is mentioned repeatedly to show the lingering attachment, indicating the deep reminiscence caused by the past things or people in memory, probably because it is precious or unique.