The human body is amazing. It adapts quickly to stresses placed upon it. This is a very good characteristic for our ancestors. Not so great for us.
Variety is key when you're trying to lose weight, get past a plateau, or become more fit. If you do the same cardio or same weight routine over and over it will become easy. This is a good thing because it means you're getting stronger, but it also means that you are burning less calories because your body is used to the demands placed upon it.
You can fix this problem by upping the intensity, duration, and speed of your workout or by changing the type of workout you do. Cross-training will recruit different muscle groups, challenge your body in a new way, and help you burn more calories.
If you always walk, try riding a bike or going for a swim. If you love the elliptical machine, try the Stairmaster. If you faithfully go to step aerobics, try yoga, pilates, or a kickboxing class. If you workout at home and need some variety, rent a new workout video or borrow one from the library.