It's common for embassies to get nervous when your reason is to visit family / spouse / significant other, as sometimes this is what overstayers do - find a way in, and then just don't leave. Therefore, your job is to assure them that:
- You're not overstaying
- You're just visiting your significant other
- You have money to cover your time there
- You have a return ticket booked
- You have valid medical insurance to cover your time there, so that you won't be a medical problem for their country
To prove you're not trying to overstay and are just visiting, you simply print out your plane tickets showing your return. If you have evidence that you're working or studying at home, a letter from your employer showing you're returning to work at a given date will help, or your uni showing the upcoming courses you're involved in.
You'll also need to print bank statements showing your current balance to show you've considered how to cover your time there. If you've only got say, $200 and are planning to stay at the best hotels when asked, it'll raise flags. They're just making sure you don't get into financial difficulty while there.
Also make sure you get medical insurance, or show intent to get it (reserved funds) and print that out, showing that you're covered for the duration of the trip.
Finally if you have an itinerary (are travelling while there) write it out, so that you can clearly show what your intended journey is, how much it might cost, and so on. A budget may even help! (Not normally required, but at this point, more documentation is good to help convince them).
You said you were confident you'd completed all requirements, but the letter clearly indicates you're missing a few things, so hopefully with the above items covered your next application will go more smoothly, and you can enjoy a trip to Belgium!