Is there a reason why Deft is so cocky?

his teamfighting? he´s arguably the best positional adc in the world, constantly dishing out dmg while keeping himself relativly save. Also he really stepped up his laning, going at least even in about any matchup. The only one that currently matches him is Imp.

and that´s a quote from the MSI-tournament in which SKT used Easyhoon for a more control-oriented playstyle that definitly had been more threatening to EDG than the skirmish/mid-centric playstyle they pulled with Faker (as EDG really thrives in skirmishes and Pawn has proven that he can handle Faker with relativly ease several times).

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He doesn't seem good to top tier ADC's in my opinion. What makes him good?

See EDG vs iG in regional qualifiers or just all EDG matches in the qualifiers.