You communicate really well with each other, perhaps better than with anyone else you've ever been romantically involved with. You're ultra-aware from the start of what each other's needs are within the relationship, and just how to respond to them. Your wonderfully strong mental connection allows you to tune right in to each other, like tuning in to a strong radio frequency.
You might not even need to talk in order to understand each other; you instinctively know what the other is thinking and how to respond to their feelings. This allows you to act as a well-functioning team, rather than as two different people with differing aims. But talking is a solid bond between you as well! You love discussing everything under the sun, anything you can think of, from silly subjects to the weightier issues that affect both your lives. Any projects or activities that require you to work together as a team are good for you as a couple -- they help you develop your relationship at the same time as acting as motivation to action. Because with a relationship like yours there is only one potential problem: you might be so comfortable together that you don't push yourselves to get anything done! But activities like playing sports together on the same team, building or renovating something together and so on are great team-building exercises that can only pull you closer together.