Our hormones also undergo entrainment. Our diurnal cycles are fixed in a way, depending on sunrise/sunset. Probably not the point of the main post but I suppose some things need to be more fixed than less fixed, whilst the grey areas can widen for some.

If it's a question of boredom, then try to separate the necessary "work" stuff from the fun stuff. Don't let the fun stuff impede progress. For example, progressive loading in the big three (bench, squats and deads) will build strength/size over time as well as increase muscle/bone density. But I don't do this for fun - it's work. I plan the workout to be productive and relatively intense and I'm out of there. For fun, I might play a game of tennis,or frisbee, or ride my bike or even go fishing.

Variety is the spice of life but truth be told, we are creatures of habit.

I recently moved my martial arts group outside when possible. I had the chance to visit with Erwan Le Corre a few weeks ago and he really inspired me to run with it.
See -http://thecommonsensewarrior.com/2010/11/09/getting-back-outside/ It gives your brain the chance work and improvise new workouts. It also keeps it new and refreshing. I also like the fact that I have to check and see what I'm going to be doing at Crossfit each day. All of the randomness in my training keeps it exciting and staves off burn out.

I for my part love randomness! ...and hate routine! That' just how I feel, but the benefits (i.e. robustness) are also a huuuuuuge point! ...so I completely agree with this!

The randomness (and resulting robustnes) makes it more than just "sth to make one feel good about oneself"! It makes "exercise" what it should be! ... i.e. something that makes one more capable of doing what one wants (e.g. anything!) and avoiding things one doesn't like (e.g. disease/injury)!

I'd suspect a little bit of both would have maximum advantages. If I eat the same thing often, my digestion seems to adapt to near perfection. However, too long at this and anything new can put it in disarray. Plus different foods will have different nutrients combinations plus potentially some nutrients that are good for us but that we don't yet know about. MIght not be wise to put all eggs in just one basket.

As far as exercise, small changes can make a difference. For instance, I hike often and thought I was in pretty good shape at least for hiking. But one time, we went to a new trail that was steeper than normal and I found I had more problems than I expected on the very steep parts. I was very fit and in shape for the moderately steep areas but not for the very steep areas! It's amazing how a slight change can make a difference! So even though I have a psychological tendency towards routine, I do try to mix it up from time to time by trying new exercises and new foods once in a while. Another advantage is that sometimes I find new things I really like that will later become part of the routine!

From a historical perspective, I suspect that paleolithic lives were also a mixture. Hunting, survival, and shelter building strategies were probably somewhat routine, but in the case of unexpected problems, a flexible mind and thought processes would have been needed in order to innovate new solutions. I suspect that is why nature made humans to prefer different strategies. Those who like routine would have been important for maintaining stability and passing on knowledge but those who like new things would have been important for dealing with new problems and innovating newer better solutions to old problems. Without new challenges, both brain and body can easily become stagnant and stunted. A rich environment means constant learning and challenge.