Since nitrogen as 3 singly filled electron in each of the outermost p orbitals, why can't the hydrogen form a sigma bond with those three and the lone pair of electrons in the s orbital just remain as it is? I think it has something to do with the positioning of the lone pair if it remains in the s orbital.
I am thinking that it has to hybridize to form an 4 sp3 orbitals so that the NH3 would have a electron pair geometry of tetrahedral. But I am basing this on the VSEPR model. But it doesn't really explain the need for the s orbitals electrons to be hybridized if we ignore the known VSEPR model. So why must it undergo hybridization?