他不当回事所以要放下
[tā bù dāng huí shì suŏ yĭ yào fàng xià]
This reflects a philosophy on letting go in relationships or personal matters, saying one should move on because the other party did not take the matter seriously ('he doesn't see it as a big deal, so you need to let go'). It implies an attitude of acceptance towards situations where efforts or affections aren't reciprocated.