Looks like handle is fairly fitted to hammer eye. You might be seeing the effects of the natural oils of the handle wood drying out, this causes shrinkage, i.e. loose hammer heads.
I usually tap the end of the non hammer head end of the handle, this uses the resistance of the hammer head inertia to help drive the handle tight in to the hammer head.
once this is accomplished, I soak the hammer head and about 2 to 3 inches of the handle in a bucket/coffee can of linseed oil overnight in a warm environment. the wood soaks this linseed oil up and swells the handle back into the head, making for a tight fit again.
You might have to do this twice but after awhile the wood will stop soaking up the linseed oil. This also acts as a preservative.