The difference in bond angles is due to the different numbers of lone pairs and bond pairs in the two species. In NH3 , the N atom has two lone pairs and three bond pairs while in H2O, the O atom has two lone pairs and two bond pairs. The repulsive interactions of lone pairs and bond pairs in water are relatively more than those in NH3 . Hence, bond angle around central atom in water is relatively smaller (104.5°) than that in NH3 molecule (107°).
