I am proud of you because of your good grades.
Ich bin stolz auf dich wegen deiner guten Noten.
Why can’t we use von instead of auf?
Ich bin stolz von dir wegen deiner guten Noten.
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Stolz sein auf is a phrasal verb. It's the same as with warten auf. There is no deeper reason why the preposition auf is correct and other prepositions aren't. You have to memorize phrasal verbs as you have to memorize prefixed verbs.
And of course, there are even prefixed phrasal verbs as aufpassen auf. It's one of the most popular prepositions for phrasal verbs in German.