they sort of explained this in their website...the creator (Shonda) and her friend has this habit of putting "MC" in their words...and according to them, when they look at Patrick Dempsey on the set...all they could think about is how dreamy he looks...hence, McDreamy. :) GREY'S ANATOMY RULES!!!
Looking back, it's easy to see when a mistake has been made... to regret a choice that seemed like a decent idea at the time, but if we used our best judgment and listened to our hearts, we are more likely to see that we chose wisely and avoided the deepest most pain regret of them all - the regret from letting something amazing pass you by.
Did you say it? 'I love you. I don't ever want to live without you. You changed my life.' Did you say it? Make a plan. Set a goal. Work toward it, but every now and then, look around; Drink it in 'cause this is it. It might all be gone tomorrow."