Why is Montresor seeking revenge against Fortunato in 'The Cask of Amontillado'?

because he insulted him

According to Montresor in his opening line: THE thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
Fortunato was vain and one night he got drunk and apparently shamed Montresor's family name.