Looks like a problem with that left front leg.  Could be the onset of arthris or an aggraveted muscle or tendon in the leg. Try to see if it is the pad, hock, or shoulder. Feel for tenderness and observe the dog's reaction to pain or pressure. That lab also looks a big heavy and coudl stand to lose several pounds. 

Does the limping or favoring of the leg happen only on the hard floor or also outside in grass?