Variety plays a huge part when it comes to fitness. When you get too comfortable in a fitness routine, you stop pushing yourself. This in turn, greatly decreases your progress. Your brain appreciates variety in everything, including fitness.
In fact, one key factor in keeping our brains healthy is a variety in social life, exercise routines, and even travel routes. So mix it up a bit, try some new exercises. Experimenting will help strengthen your mind and boost your motivation.