The 25-year-old striker has been called up to the Spain squad for their friendly international against Italy at Atletico Madrid's Vicente Calderon on Wednesday.
It is the first time that Costa has the chance to play for Spain since committing his international future to the European and world champions in October 2013, despite featuring in two friendly matches for Brazil.
The Brazilian football federation has criticised Costa's decision and there have been reports that some members of the Spain squad are not in favour of the idea either.
However, Costa claims that much is being made about his wish to represent Spain and insists that he has 'earned' his place in the squad.
"I never imagined [all the fuss]," he told Canal+. "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.
"(Barcelona defender and Brazil international) Daniel Alves never tried to convince me. Not him, not anyone in Brazil. The call-up by Brazil was a dream, it was a unique moment.
"(Luiz Felipe) Scolari called me up because he thought I was a useful player, not to stop me playing for Spain."