To a psychologist, lots of human rituals look a lot like the automatic behaviours developed by Skinner's pigeons or Dickinson's rats. Chunks of behaviour that do not truly have an effect on the world, but which get stuck in our repertoire of actions.
We cling to these habits because we – or ancient animal parts of our brains – do not want to risk finding out what happens if we change. The rituals survive despite seeming irrational because they are coded in parts of our brains, which are designed by evolution not to think about reasons. They just repeat what seemed to work last time. This explains why having personal rituals is a normal part of being human. It is part of our inheritance as intelligent animals, a strategy that works in the long-term, even though it clearly does not make sense for every individual act.
Stafford T. Sporting superstitions: Why do we have them? 27 March 2012 [interactive] [accessed 05 April 2012] http://www.bbc.com/future/story/20120327-why-do-we-have-superstitions/2