Why was the Maastricht treaty so hard to pass?

For lots of people, the Maastricht treaty was the first real step into the concrete future of the European Union. The Maastricht treaty represented obligations and responsibilities for EU member states and took away political sovereignty in some areas. For nations whose people had only signed up to the EEC (previous name of the EU) to be part of the Common Market, this was worrying because they would no longer have control over certain parts of their laws.
The nations most concerned by Maastricht (the UK, the Netherlands, Denmark) fought most of their battles in the EU's negotiation rooms before the treaty was actually signed. However, Denmark's constitution said that anything that risked taking away sovereignty had to be subject to a referendum- so Denmark put the Maastricht treaty to a referendum and they lost. Because big European treaties need unanimity (everyone needs to say yes), Denmark agreed the Edinburgh Agreement the next year (1973), where they would get 4 major concessions on future integration on the EU. The referendum including these concessions passed and Maastricht passed into law the same year.