世上没有不分手的情侣只有不努力的小三
[shì shàng méi yŏu bù fēn shŏu de qíng lǚ zhĭ yŏu bù nŭ lì de xiăo sān]
'There Are No Couples Who Won't Break Up, Just Not Enough Effort from Third Parties.' implies every relationship faces challenges from external forces (often referred to as 'third parties'). It cynically refers to how couples breaking up could sometimes be influenced by a third party, or infidelity.