... trotting out of his herd to me as soon as he recognizes me.

As he does, I press the shutter release of my camera so that it takes a sequence of many photos per second while Shane approaches me at a quick pace. Here are some photos taken from that series and placed alongside one another. For comparison, the first photo appears again at the end.

On the first photo the gray seems happy, eager, and full of anticipation. 
In the three following photos, Shane’s expression changes. 
The horse’s lively look disappears.

Why?
What happened?

Click on the first photo to study the pictures more closely.
Run your mouse over the photos to read the text concealed behind them.