Getting too relaxed with your exercise routine can not only get boring, but can also put your health goals on hold. It's easy to reach a plateau when your exercise becomes routine. Some believe to maintain the maximum benefit you should change your routine daily or every couple of weeks. Still there are others that believe and make valid points that you really should not change your routine.
I think that it depends where you are and where you are going. If you need to lose weight because you are overweight, and are at a stand still, then by all means change your routine. If you want to build more muscle and are under 50, then by all means change your routine.
At some point it is more about maintaining what you got rather than taking it up a notch. You run the risk of overtraining and tearing up your muscles, if you keep uping the ante, you can do more harm than good.