The prolific Atletico Madrid striker pledged his allegiance to the world and European champions in October and is set to make his debut for his adopted country against Italy on Wednesday.
Costa, 25, made two appearances for Brazil in friendlies last year, but is now part of Vicente del Bosque's Spain squad ahead of this year's FIFA World Cup in Brazil.
Much has been made of the forward's decision to snub Brazil and their coach Luiz Felipe Scolari criticised Costa for "turning his back" on his country of birth.
But Costa is in no doubt he made the right choice as he prepares to showcase his talents on the international scene once again.
He told Canal+: "I never imagined (that his decision would attract such attention).
"When I realised there was interest from Spain I started to imagine things, and thought 'why not?'.
"It is a privilege that the world champions want you to play for them, especially given the quality of players they have.
"I felt very important. I value it a lot. Vicente del Bosque showed me the person that he is. I like to talk with people face-to-face, I feel the truth and lies, and Vicente was very clear.
"He didn't promise me anything. I don't like it when people promise me things, I like to earn it. This I value."
Costa has scored 27 goals in all competitions for La Liga title contenders Atletico this season.
