It is shown that the first law of thermodynamics and the holographic principle applied to a cosmic causal horizon demand the zero cosmological constant and non-zero dynamical dark energy in the form of the holographic dark energy .
This dark energy has a parameter d = 1 and an equation of state w _ { 0 } \simeq - 0.903 comparable to current observational data , if entropy of the horizon saturates the Bekenstein-Hawking bound .