Boggarts se alimentam de medo e se fortalecem com isso.

Boggarts se alimentam de medo e se fortalecem com isso. Portanto, a razão pela qual eles gostariam de criar medo seria para que eles pudessem se alimentar disso. Na J.K. Rowling escrevendo sobre Pottermore sobre Boggarts, mencionou que os Boggarts parecem ser criados e sustentados por emoções humanas.

Boggarts can be made to disappear, but more Boggarts will inevitably arise to take their place. Like poltergeists and the more sinister Dementors, they seem to be generated and sustained by human emotions.
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Famous Boggarts include the Old Boggle of Canterbury (believed by local Muggles to be a mad, cannibalistic hermit that lived in a cave; in reality a particularly small Boggart that had learnt how to make the most of echoes); the Bludgeoning Boggart of Old London Town (a Boggart that had taken on the form of a murderous thug that prowled the back streets of nineteenth-century London, but which could be reduced to a hamster with one simple incantation); and the Screaming Bogey of Strathtully (a Scottish Boggart that had fed on the fears of local Muggles to the point that it had become an elephantine black shadow with glowing white eyes, but which Lyall Lupin of the Ministry of Magic eventually trapped in a matchbox).
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Portanto, a razão pela qual os Boggarts se transformam nos piores temores das pessoas é para que eles possam criar a maior quantidade de medo possível para que eles possam ter mais medo de se alimentar.