Jamieson, Fausset and Brown on Matthew 10:39

39. He that findeth his life shall lose it: and he that loseth his life for my sake shall find it--another of those pregnant sayings which our Lord so often reiterates (Mt 16:25; Lu 17:33; Joh 12:25). The pith of such paradoxical maxims depends on the double sense attached to the word "life"--a lower and a higher, the natural and the spiritual, the temporal and eternal. An entire sacrifice of the lower, with all its relationships and interests--or, a willingness to make it which is the same thing--is indispensable to the preservation of the higher life; and he who cannot bring himself to surrender the one for the sake of the other shall eventually lose both.