It's one of the most common mistakes people make when cooking a piece of meat either on the stovetop or the grill. In an effort to make sure that the meat is cooked evenly they turn it too often.
Ideally, a piece of meat should only be turned once: you cook it on one side and then flip to the other....no more, no less.* Reason being that every time you flip a piece of meat back over to a side that's already been cooked the outer layer of meat gets cooked yet again before the heat ever makes it towards the center. What results is a tough, overcooked exterior. This is especially true if you need a good crust.
*This assumes you're cooking a two-sided piece of meat such as a burger patty. If you're working with a larger piece that needs searing on more than two sides just make sure to cook it once on each side.
Also along those same lines it's worth noting that you should never use a spatula or tongs to press down on a piece of meat. This is quite a common practice (prior guilty party myself) that does nothing other than squeeze out the juices that keep your meat moist and flavorful.
This is one of those situations in which patience is a virtue. Just let the meat do it's thing...it will be well worth the wait!