心是你的狗粮
[xīn shì nĭ de gŏu liáng]
The expression uses 'dog food' as a metaphor to show a kind of envy, possibly stemming from witnessing another's relationship happiness. The sentiment behind 'hearts being dog food' likely implies the pain of unrequited love, seeing what you want yet out of reach. This idiom comes from the Internet culture in China which originated from people jokingly calling couple photos as 'dog food.'