He told Marca: "I look back and think that I was completely right with the decision I made. If I were not here I doubt I would have reached this level."
Having overcome two serious knee injuries in the past, he admits harbouring doubts, but Romeu has been one of the most-utilised players by Saints boss Puel.
The French tactician has told the 25-year-old Saints star he can go on to represent his country at senior international level under Julen Lopetegui - his former Spain under-21 boss.
"I was with Julen for a year in the Under-21s and enjoyed it," he added.
"I am playing well, I don't know if I can make the squad, but it is clear that we all dream of playing for Spain and I will continue working for it."