Over time one is wont to forget things as they happened and remanufacture them in memory as one hopes they happened. At points along the way, however, one remembers things just as the were as opposed to how one may have wanted them to be at the time. The denominator most common to both cases is that the memories that come to us as happy ones are often altered and the ones that are unhappy ones are generally unaltered. A depressing thought but I suppose for survival's sake it makes a bit of sense