There are three main reasons meat will be tough:
1. Connective tissue (silver skin) - Connective tissue is a protein based structure, that is found inside the muscle and between the muscle and the skin. When it comes to areas with a high density of connective tissue you’ll notice the ‘silver skin’ on top of the meat. Although trimming this off with help with the tenderness of the meat, it will not solve the issue because most likely this is intramuscular fat and goes throughout the entire piece of meat. The best way to handle a cut like this is to slow cook it. Think of a brisket being smoked for 12 hours, the goal is to render the intramuscular fat to make it become tender. If you cook at a high temperature for a short duration the meat will stay tough.
2. Contracted Muscle - Processing an animal before it has reached rigor mortis will cause the meat to contract and become more tough. This is the idea of cooking an animal immediately after harvesting will lead to a worse end product. After the process of rigor mortis has occurred the meat will not contract when exposed to heat or being cut. Meaning, it will be much more tender.
3. Fat - The main aspect when it comes to fat is the amount of marbling. Marbling is the foremost factor when determining the grading of meat. This is how the USDA makes its grading scale for prime, choice, or select cuts, but that's a different story for another day.