Several factors influence this, the most important are:
1. Storage Method
2. Type of coffee (ground or whole bean - the more ground, the more likely it will be bitter)
3. Water Hardness
4. Mineral composition in the water this has a big influence on the chemical reactions that occur during coffee brewing, therefore it may be worth filtering the water before using it to make an espresso
5. Water that is too hot (the correct temperature should be between 185 and 205 degrees Fahrenheit).