Because losing weight requires "change." What's interesting about change is sometimes it's hard and sometimes it's not. There are many changes we embrace that are extremely difficult: having kids, changing jobs, moving, divorce... but we manage to do them and survive - even thrive.
So the difficult part is how you view things headed into it. If you look upon your weight loss as an opportunity, a chance to improve and feel better - then you are off on the right foot. If you start out already thinking in terms of deprivation, denial, and punishment - then you will have a rocky road.
If you have the right attitude, view it as a problem to be solved in no way tied to your self-worth, add a little activity and cut back on some food... you'll rock the house.