相思泛滥成灾
[xiāng sī fàn làn chéng zāi]
It refers to overwhelming lovesick, longing thoughts have multiplied out of control to the point of becoming like floods that cannot be restrained anymore—it implies having very intense affectionate feelings which spread everywhere just like floods causing damages in its path; it could also refer to being deeply immersed in romantic fantasy daydreams.