The little dog is having a bit of a rough time right now. She's always been sure she's Maisie, but (and those of you with multiple pet households will understand this next part) she's also been pretty sure she's Jake. This post from April 2008 says it best:

She's always in the lead, little Maisie. She's smaller than Jake so she's faster and more nimble. But other than when she's chasing the ball, a moment in time I'm convinced she thinks she's a wolf running down some helpless prey (though this particular wolf has short tiny legs, big puppy eyes and has developed a bit of a belly) she makes sure she never gets too far ahead.

It's like she's got some calibrated graph in her head, she's X, I'm Y and Jake is the axis on which Maisie sets us. She nudges and herds him and, when she can't stand for one more minute that he's NOT OBEYING, she just grabs him.

So here, though you maybe can't tell, she's keeping a precise distance. Jake never gets too far behind and, most important to Maisie, he's positioned so she can whirl around and clamp her tiny jaws around his neck at a moment's notice.