The two most likely possibilities are: 1) excessive spring tension or 2) incorrect installation.
Door closer spring tension is either adjustable or fixed. To find out which kind of closer you have, you'll need to identify the make and model and then download the install instructions from the manufacturer's web site. If the closer has adjustable spring tension it will say so in the install instructions and also tell you how to adjust it. If your closer has no spring tension adjustment the install instructions may show how to locate the closer differently on the door to decrease the tension. Also, using the install instructions and a tape measure you can tell if your closer is installed incorrectly. If it is installed incorrectly this could also affect the spring tension.