There are a couple of interesting points here depending on the precise meaning.
Firstly there is something called T symmetry
http://en.wikipedia.org/wiki/T-symmetry
the T here is time and the principal is that a process that works one way in time should be able to work the other way too.
This symmetry is true only in restricted areas - it fails in quantum mechanics and in terms of entropy.
Entropy is a measure of the disorder in a complete system and the second law of Thermodynamics says that overall Entropy is always increasing and that things are always getting more disordered.
Why that is so is indeed something that has occupied a lot of great minds but have a look at this:
http://en.wikipedia.org/wiki/Arrow_of_time
for an overview