When von Aschenbach sees the strange, young, foreign-looking man, there is a change that comes over him which makes him want to travel. This was not Aschenbach's normal custom, for he usually only left Munich for his health, not for pleasure. The decision proved to be a fatal one, for it leads to Aschenbach's death.
He was brought back to reality by the sight of a man standing in the portico, above the two apocalyptic beasts that guarded the staircase, and something not quite usual in this man's appearance gave his thoughts a fresh turn.
... He was of medium height, thin, beardless, and strikingly snub-nosed; he belonged to the red-haired type and possessed its milky, freckled skin. He was obviously not Bavarian; and the broad, straight-brimmed straw hat he had on even made him look distinctly exotic. True, he had the indigenous rucksack buckled on his back, wore a belted suit of yellowish woolen stuff, apparently frieze, and carried a grey mackintosh cape across his left forearm, which was propped against his waist. In his right hand, slantwise to the ground, he held an iron-shod stick, and braced himself against its crook, with his legs crossed. .... At any rate, standing there as though at survey, the man had a bold and domineering, even ruthless air, and his lips completed the picture by seeing to curl back, either by reason of some deformity or else because he grimaced, being blinded by the sun in his face; they laid bare the long, white, glistening teeth to the gums.
Aschenbach's gaze, though unawares, had very likely been inquisitive and tactless; for he became suddenly conscious that the stranger was returning it, and indeed so directly, with such hostility, such plain intent to force the withdrawal of the other's eyes, that Aschenbach felt an unpleasant twinge and, turning his back, began to walk along the hedge, hastily resolving to give the man no further heed. He had forgotten him the next minute. Yet whether the pilgrim air the stranger wore kindled his fantasy or whether some other physical or psychical influence came in play, he could not tell; but he felt the most surprising consciousness o a widening of inward barriers, a kind of vaulting unrest, a youthfully ardent thirst for distant scenes -- a feeling so lively and so new, or at least so long ago outgrown and forgot, that he stood there rooted to the spot, his eyes on the ground and his hands clasped behind him, exploring these sentiments of his, their bearing and scope.
True, what he felt was not more than a longing to travel; yet coming upon him with such suddenness and passion as to resemble a seizure, almost a hallucination. (4-5)
The slightly hostile air of the stranger can be seen in different ways; the youth, regarded by this old man, does not want this attention and makes that plain. This could be a prefiguring of the lack of interest Tadzio will have in Aschenbach later in the book. The foreign look of the red-haired man, too, is ominous. While the "pilgrim air" is at first intriguing, the pulled-back lips and the hostile attitude of the young man can be seen as a warning to Aschenbach to not leave home, with its familiar things and people. Aschenbach, a solitary man, spends a great deal of time thinking about himself. He becomes ruled by his whims, which causes his downfall.
Source: Mann, Thomas, Death in Venice and Seven Other Stories. H.T. Lowe-Porter, trans. New York: Vintage, 1963.