Murphy's Law (otherwise known as the Law) states that everything that can go wrong, will go wrong. From a scientific point of view, this may be stated as, "Out of two or more possible outcomes, whichever option which will cause the most damage will happen." Suddenly needed objects become invisible, buses always come in threes, and the toast always lands butter side down. This is a result of many (toasty) experiments.
Most of Murphy's law relies primarily on chance. Theoretically, your toast will only land butter side down half the time - it's just that the human mind is more pessimistic than optimistic, and only notices when the bad things happen. The glass is half empty, not half full. If you look back on your life, what do you remember more vividly? The good times or the downright embarassing times?
Unless you're Ned Flanders, you'll say you remember the bad times more.