Here's why:

* Nitrogen's electronegativity: Nitrogen is more electronegative than hydrogen, meaning it attracts electrons more strongly.

* Covalent bonds: The bonds between nitrogen and hydrogen in ammonium are covalent, meaning they share electrons. However, the shared electrons are pulled closer to the nitrogen atom due to its higher electronegativity.

* Formal charge: This unequal sharing of electrons results in a formal charge of +1 on the nitrogen atom.

* Overall charge: Since the four hydrogen atoms each have a neutral charge, the overall charge of ammonium is +1.