
My lab has always done this. I have to say that going to obedience lessons has helped no end because he will now bring back and object to me if I say he is a clever boy and call him to me. He just places it in my hand. I have also been doing working trials with him and I often put "naughty" objects out for him to retrieve. You know the type of things like a 3"x3" piece of scouring pad or a pine cone or a cork, something that a lab would love to swallow!!! but through training he has learnt to hold them and give them to me.
When he was a puppy, we used to do swaps...i.e he picks up a stone, clothes peg etc and I swap him for a treat. You could try that for a quick response?