Have you ever had a case of the grass is greener on the other side?
With the horses, rather than any sort of pining for something they cant have, it seems far more likely that it is simply the fact that being ungrazed, the grass definitely is longer and greener, and thus requires less effort to get a full belly. Evolution will have strongly selected for neural networks than deliver full bellies with least effort – hence the attention on the grass that is longer (and evolution will not have encountered fences over deep time, they are a very recent invention).
Lots of other complicating factors in there also. Evolution probably selected quite strongly in wild horses for grazing and moving on, to reduce the risk of picking up some disease from the leavings of other herd members. We haven’t been keeping them in paddocks long enough to have entirely selected out such behaviours.
And sure, there will be a curiosity factor, and a nervousness factor, both of which will have been determined by the long term balance of risks and rewards of such behaviours over deep evolutionary time and the variety of contexts encountered over such time. And all the many other aspects of behaviour we see in complex animals like us and other mammals.
And we humans are much like that too.
Our tendencies to different types of behaviours are very much determined by context, though we bring added dimensions to the social and intellectual aspects of our contexts and behaviours.
Recursion is a marvellous thing, being part of both abstraction and creativity, at potentially infinitely extendable levels.
Any level of any infinity can be explored for infinite time, so all of us who are involved in exploration of anything at any level can be said to bring a “grass is greener” aspect to our being.
We really are “spoilt for choice” as the old saying goes.
And every new level has its own sets of risks, and requires the development (ongoingly) of effective risk mitigation strategies.