Basically, in Operation Trinity, we find out that Ian-stalking-Amy is actually canon. So, here's a little something based on that. :)
An Unusual Hobby
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He is not obsessed.
It's simply a hobby. Everyone needs a hobby, right?
It is a bit strange, he'll admit that, but the American government does worse to suspected terrorists. He's certainly not tapping phone lines (though he definitely could), or digging in trash (though he could hire someone to).
He's simply watching—but not constantly, of course. He does have other (more important) duties to attend to.
Although, his hobby does take up quite a bit of time, especially with the new, and unwelcome…arrival. He has to make sure this intruder doesn't cause any harm to his subject or mess up his perfect system.
She ("the subject") is an integral part of his family and if she were damaged…, well, it wouldn't be good. His relatives seem to think she (and her brother) is their last hope, and, as much as he wishes he could believe otherwise, he agrees.
That's all this is, after all, a hobby turned familial obligation. Because family should look out for family, right?
Sometimes, if he's in town, he follows.
It's a necessary action, because the intruder takes her places far from his camera lenses and he needs to make sure she stays safe. For their family's future, of course. He's certainly not jealous of the intruder.
Not jealous at all.
…so maybe there's a few drops of envy permeating in his veins when the intruder makes her smile, or holds her hand, or kisses her on the porch steps.
But he's not jealous.
He knows that the intruder is anything but permanent.
Soon she'll realize that, too.
For now, he watches, and follows, hoping to catch her without anyone by her side. Hoping to see her with more library books than she can carry, happily struggling to keep them balanced. Hoping to catch her smiling at the insignificant details she loves so much, like the swirling leaves in the summer breeze.
...He is not obsessed.
(He just likes it best when she smiles.)