I'll give you the answer child me would say: presents and cake.

In other words, we celebrate because we get something desirable out of the celebration or the act of celebrating.

Writer, philosopher Alain de Botton: "We should cite our age not by how many years we've lived, but by how many we statistically have left: a 70 yr old UK male as 8 etc."

http://twitter.com/#!/alaindebotton/status/95809899929407488

There is a few other planetary orbital periods in there if you prefer another arbitrary unit.

And other bases too incase you weren't born with 10 digits.