The Atletico Madrid forward, top scorer in La Liga this season, signed a formal letter on Tuesday declaring his availability to Spain coach Vicente del Bosque, a decision that prompted an angry reaction from Del Bosque's Brazil counterpart Luiz Felipe Scolari.
"It was a very difficult decision but everything I have achieved in my life, everything has been given to me by this country (Spain)," Costa said on the world and European champions' official website (www.sefutbol.com).
"I feel extremely valued here for everything I do, for the work I do on a daily basis, and I feel people's affection," the 25-year-old added.
"I devoted a lot of thought to the decision and I hope people respect it and understand that it's not a rejection of Brazil."
Costa's decision led Scolari to withdraw the player's invitation to take part in friendlies against Honduras and Chile next month.
He must appear in an official competitive game, such as next year's World Cup finals in Brazil, and not a friendly to be tied to one nation.
"A Brazilian player who refuses to wear the shirt of the Brazilian national side and take part in the World Cup in his own country must automatically be uninvited," Scolari said.
"He is turning his back on the dream of millions, the dream of representing the five-times world champions at the World Cup in Brazil."
However, the reaction failed to deter Costa and he said playing for Spain would be "an honour for any player".
"If I only play five, 10 or 15 minutes, whatever it may be, I will give my all because I have always played that way for all the teams I have been part of," he added.
(Reporting by Iain Rogers, editing by Alison Wildey)
