Perhaps only a man as cold as Wagner could write what Tristan became. Only one that could have a rational "coldness" in his shoulders to have the courage to the able to write about the true depth of love. Perhaps he never felt as deep as Tristan felt.
On the other hand, and in a lighter music, Jacques Brel must have really felt to be the scum of society to write Ne me quitte pas. I think that when you are the same situation as he was, you can, then, truly understand about what he wrote.
And back to opera again, we have Verdi, who describe human feeling, as they really are, with music. Each note of his in, let's say, Rigoletto reflects the exact emotion in every part of the action. I don't think also that he had to suffer to compose Cortigianni vil razza. He was a great observer of the world, a very meticulous painter, a precise writer that could describe exactly every moment with the proper words.