Very nice dog, one of my favorites!
My uncle had a boxer that looked identical to this one, but it was a male. He took him in as an abused dog from a dog shelter. He lived a very long time for being a large dog and having a rough first year of life. They were completely inseparable and my uncle was a complete wreck when he wasn't around anymore.
About two years ago my uncle got a brindled boxer, this time a girl, and it too was an abused dog that he rescued from a dog shelter. She absolutely will not go around strange men (she's even anxious about going to my uncle at times), especially if they have black hair (the abusive owner must have had black hair is all we can guess).
Oddly enough, the first time I met his new boxer, she walked right up to me and nuzzled her head against my leg... and I have black hair too. My uncle was totally shocked, and she still won't go to any other guys with black hair except for me. Go figure.
-JT