Here's why:

* Brønsted-Lowry Acid-Base Theory: This theory defines acids as proton (H+) donors and bases as proton acceptors.

* Conjugate Pairs: When an acid donates a proton, it forms its conjugate base. Conversely, when a base accepts a proton, it forms its conjugate acid.

Example:

* Ammonia (NH3) is a base. When it gains a proton (H+), it forms ammonium ion (NH4+), which is its conjugate acid.

Equation:

NH3 (base) + H+ (proton) ⇌ NH4+ (conjugate acid)