Why?

Why will an American robin stay put when my dog runs by?  Why will this robin hold it's ground as I approach anytime I've left my camera behind?? (Seriously, I can get within 10 feet of one...)  Why will any of these robins disappear immediately if I'm anywhere in the area with my camera on me???  

I got lucky and caught this one by surprise today...  When he spotted me, you could practically hear him say, "Dang!  She caught me off my guard!!"  Then he promptly fled the area.