When we read a literary work (or, in some instances, listen to music) our imagination is stimulated, we feel various emotions, and we arrive at new judgments. These attitudes are brought into relation with many others, including our standing tendencies to think and feel in particular ways, and we try to fit our psychological capacities and responses together.
Sometimes, however, the new synthetic complex proves stable, and even serves as the beginning of a much larger cluster of attitudes that displace some we've previously considered to be fixed parts of ourselves.
The amalgam of psychological attitudes we form is the synthetic complex. It may fall apart quite quickly as further reflection or further experience bears on it, and we may revert to our former judgments, feelings and tendencies.
Mann's sexuality and his attitudes towards it are extremely complex - and the complexities are inherited in the figure of Aschenbach. Mann had lived through a series of (almost certainly unconsummated) relationships with young men.
