There is another similar scene in the movie as if Jack couldn't get the horse to move except to "startle" around.
Ennis had that grin on his face. "See I told you so." 
I guess Jack sees Ennis as a "problem-fixer" while he is the "idea-generator". Both of them would make a great team.
There are a few horsey type comments that I think people didn't quite get. Here Ennis says, "That mare has a low startle point." This means she startles or spooks easily, so don't get her riled up. She could throw you. I take this to mean that he is thinking, "Look at this ass. He's messing aound with the wrong horse," in a cute amused way.
When Ennis is putting up the tent and says, "That harmonica don't sound quite right." I don't recall Jack's exact words, but some people have quoted him as saying he got thrown or something to that effect by Mary. He is talking about the horse, a "mare", a female horse. I thought Ennis then said, "I thought you said that mare can't throw you." VERY FUNNY! Jack is laying down with his head on a log and his hat down over his face. When Ennis says this, we see a huge grin under his hat. It's adorable.
I also found amusing:
Acguirre: If you two dueces are looking for work, I suggest you get your scrawny asses in here pronto.
I didn't like this comment. I think their asses are cute not scrawny, but the way the two of them shuffle in like two puppy dogs with their tails between their legs makes me smile.