On the basis of spin dimer analysis and density functional theory electronic band structure calculations, we examined why the magnetic ground state of SeCuO3 is ferromagnetic while that of its isostructural analogue TeCuO3 is antiferromagnetic and estimated their spin exchange parameters. The essential difference between the magnetic properties of these oxides arises from their Cu-O(1)-Cu superexchange, but not their Cu-O(2)-Cu superexchange. Spin exchange paths relevant for understanding magnetic properties are those that contain magnetic orbitals.