The connection between microscopic, time reversible equations, and irreversibility of macroscopic systems is discussed. In particular, the questions (i) why does a system reach an equilibrium state and (ii) why does it remain indefinitely in that state are addressed through a discussion of the microscopic description of a macroscopic system. The origin of irreversibility can be elucidated considering a system interacting with some external reservoir, leading to a Langevin equation that incorporates the effect of the neighborhood of the system through a friction and a random terms and consequently irreversible behavior.