To mourn in this context means, to bemoan oneself, or to express grief for not having what one should have. (Matt.5:4)

Jacob bemoaned himself for the ‘loss’ of Joseph (Gen.37:31-34). Esau did so for the loss of his birthright (Gen.27:34-38).

As one mourns for one’s lack, one is simply expressing the invaluableness of what one is lacking, as well as expressing the need for such things. God commands this attitude at times of need (cons. Jer.4:7-8; Jas.5:1-3)