Because this dog is very smart and gentle, it's a good friend to me.

Literal

Extremely clever and-obedient [because-ので], this dog [topic-は] me [for-にとって] good friend [copula-です].

Two adjectives joined with the te-form: 賢く ('clever') + おとなしい ('gentle/obedient'). ~にとって means 'for me, from my perspective.'