Why is absolute zero the lowest possible temperature?
By definition, absolute zero is the lowest possible temperature that can be reached - nothing can be colder. All of the particles of a material at absolute zero have their minimum possible amount of energy (known as the zero point energy).
It can be shown from the laws of thermodynamics that absolute zero can never be achieved. Simply put, it is impossible by any procedure to reduce the temperature of a system to zero in a finite number of operations. (≈ Guggenheim).