"Strictly speaking, impulse control is a misnomer. You don't really control the impulses. even someone as preternaturally disciplined as Barrack Obama can't avoid stray impulses to smoke a cigarette. What he can control is how he reacts, ignore the impulse, chew a Nicorette or sneak out for a smoke," (Baumeister and O'Hare, Willpower)
Willpower is like a muscle. Though it is a cognitive process and takes place within the brain-the more you work it, the stronger it becomes. However, willpower has a limit. Just like a muscle, it can fatigue and fail.
This concepts ties into my post 'Dog Training from an Athlete's Perspective.' Willpower, what it means for our dogs and how to get more of it! You may remember about how some stress is good, it builds confidence and resilience for future difficult situations. But too much can be discouraging, overwhelming and cause us (and our dogs) to to give up.
Mental work is fatiguing. Even though your day at work may be done sitting in a comfortable chair, those hours of spreadsheets are exhausting. When we're even partially depleted we notoriously make bad decisions. One study found that those on diets (working mentally to not eat what they crave) are actually more likely to cheat on their spouse! Social scientists also demonstrate this in laboratory settings by asking participants either resist a temptation, or control emotions - then having them work on a simple puzzle. Those who had to use willpower to not laugh at a comedy, eat the chocolate cake, etc give up much sooner on these puzzles.
The squirrel will also be alluring - just as that hazelnut soy latte is always alluring to me. What matters though is the choice we make in response to that craving. I can't avoid driving by Starbucks, but I can practice not going into every one. Similarly with our dogs, so much of what they bark at, eat, chase after and play with can't be avoided. So rather than attempting to employ an almost never ending 'leave it' its important to teach your dog how to actually handle those unavoidable impulses. Set them up in situations which they can handle, so they are not overwhelmed and fail. Over time, their ability to control their response to those impulses will improve allowing them to respond well to greater and greater distractions.
My puppy Tango has the unending desire to put everything in his mouth. Socks, shoes, my hands, rocks, power cords.... Living in a studio, it is unavoidable that these items are left out in places he can reach. To teach that these were things to leave alone, I started with just one object at a time. While it was out, if he showed any interest, I initiated play with an appropriate toy, or called him away and gave a treat. At 6 months old as I type this, my shoes, a bag, some clothes and a few other things are out and haven't been paid any attention. He still has the impulse to chew, but he now diverts it to appropriate toys.
Dogs will be dogs! Temptations are always out there. Set your dogs up for success by proving them with the opportunity to make the right choice in situations they can handle. Help them with incompatible behaviors (they can't jump on a guest if also in a sit/stay), and give them frequent breaks. The more they can practice willpower over impulses without depletion, the more second nature self control will become.