This is an extremely nearsighted view of the bloc.
Things that benefit the EU will benefit my country too. Things that make Romania or Bulgaria worse will also impact the other countries in the bloc. I thought we learned this lesson when countries like Greece had fiscal issues back in the day.
Politicians at the EU level should be concerned by their country, but also should be concerned about the bloc as a whole.
And this is true to national politics too. A member of a national parliament typically is concerned with the province/county/constituency he represents, but also with national issues as a whole.