It's all down to the proportions of water, salts, muscle, proteins, sugars, acids, oils and fats in an animal at the end of the day.
To begin with some meats, per se, have a uniquely distinctive taste even before cooking. Fish is the obvious example.
Other factors such as the type of feed an animal consumes, it's living environment and whether or not it's lived under stress have been shown to play a part. For example, corn-fed chickens are said to taste better than battery chickens.
Animals that are stressed tend to produce more adrenaline than those that are not, and this has been shown to have a deleterious effect on the quality of the meat. Bovines in particular are prone to this problem much to the dismay of abattoir owners up and down the country as organic meat from animals killed humanly are said to be more flavoursome..
Turning to cooking, meat proteins denature during cooking and the denatured protein combines with sugars causing the meat to brown. This browning reaction is called the "Maillard Reaction" and it the same reaction that occurs during bread production - it's actually a reaction between a reducing sugar in a carbohydrate and an amino acid, called lysine, in a protein.
Following slaughter, the proteins in muscle tissues of animals is broken down into smaller proteins called peptides. These peptides contain amino acids and certain combinations of them can powerfully enhance the flavour of meat. It's so powerful that the peptide mixtures are sold nowadays for flavour enhancing of budget meat cuts.
Muscle tissue contain substances that produce and lock-in flavouring molecules. This is why the muscular legs of many animals have tastier meat than other parts of the same animal.
Fat also soaks up flavours during cooking and a beef joint will hold on to more of the flavours after roasting than leaner poultry would. This is the reason why beef