I can agree with the above as far as some reasons. Also, boxers especially, dogs are smart. Often times it's just a game to them. They figure out you will chase them to get the object back. So they learn that it's the trigger for a game with you.
Try grabbing a toy, call the dog and then removing the object and replacing with the toy. Play a short game of fetch or tug to get interracted with the dog. Teach him toys will get a much better reward than stealing your things.
I will add, my one rescue will steal socks, and my pillows. He doesn't do anything but lay with them. I see this as a stress coping behavior. But often my pit/boxer will them try to take the item to intiate a game of tug, therefore the object is often destroyed! LOL