I think we lose some sense of patience, plus with kids, work life etc it is all so hectic so when someone is messing around or not doing things the way you would hope I find myself getting a little tetchy since I've got better things to do.
Plus we have learnt / developed our tastes better so we know what we want and can't be bothered to put up with sub-standard stuff normally - hence we appear more 'grumpy'