Why do due dates still bother me?

At first it was subconsciously. I was emotional, angry, irritable, missing the twins and felt that tugging at my heart that is the longing to be a mother.. Then I realised it is nearing what would have been the twins due date Their due date was March 11th 2009 they were due on this date, and it was 2yrs ago!.. This will be the 3rd of their due dates that I have been through.. I truly have no clue why it effects me so much.. Most bubs aren't even born on their due date!Anyway, today it's just the same story. I'm unhappy, I wish my babies were here,I feel unfulfilled.. Feeling like shit today.. Blah..