I think it is quite self-evident if you read the text carefully that the passion that von Aschenbach has for the Polish boy Tadzio is based on lust rather than love. Clearly the feelings that von Aschenbach has for Tadzio are not based on self-sacrificing love. Consider the following example, when von Aschenbach follows Tadzio and his sisters as they take a gondola:
Leaning back among soft, black cushions, he swayed gently in teh wake of the other black-snouted bark, to which the strength of his passion chained him. Sometimes it passed from view, and then he was assailed by an anguish of unrest... Yes, this was Venice, this was the fair frailty that fawned and that betrayed, half fairy-tale, half snare; the city in whose stagnating air the art of painting once put forth so lusty a growth, and where musicians were moved to accords so weirdly lulling and lascivious. Our adventurer felt his senses wooed by this voluptuousness of sight and sound, tasted his secret knowledge that the city sickened and hid its sickness for love of gain, and bent an ever more unbridled leer on the gondola that glided on before him.
It is clear that there is nothing pure in this love. Note how von Aschennach is apparently inspired by the decay and corruption and lust of Venice to give an even more "unbridled leer" towards Tadzio. Note too that he is described as being "chained" to the gondola and following the object of his affection by "the strength of his passion." Arguably, love is not something that enchains us but liberates us. Again and again in the text, von Aschenbach shows that he is actually ensnared by his passions, strongly indicating that they are based on lust and not love. In Venice, it is clear that von Aschenbach has fallen into the "half-fairytale, half-snare" of the city that has beguiled so many others before him.