One big reason why a country would like to join the EU is the open borders, especially from economic point of view. The member countries of the EU do not pay tariffs between each other, thus the goods are moving freely. This is something that every country wants, as it enables its economy to thrive, its companies/corporations to be more competitive, and the country to be developing.
One big reason why a country wouldn't like to join the EU is that it loses part of its sovereignty, a big one. The EU has its own leaders, its own parliament, its own rules, and laws. Al member countries are obliged to act in accordance to them, not as they will. That may be a situation that will make the leaders and people of a country do they want to be controlled by someone else or not.
