You reinforce your partner's ideas of the direction their life should take, which is a relief for them, to say the least! It can be difficult sometimes to constantly push forward through life, and that makes it extra nice to find a partner like you are for them -- someone who encourages them and understands them, and helps them find their way. They feel as if you're their safe harbor, their teacher and guide, their inspiration and their best cheerleader.
And they return the favor by serving all those roles for you, as well. Something about accompanying them on their journey actually helps you along yours. You help each other out on a regular basis by giving each other advice -- and it's sound advice, because you understand each other so well that you really have a good idea of where your lover needs to go next. The level of respect between you really helps you listen to that good advice and take it when appropriate! There is a feeling between you that you're traveling life's many paths together, and that helps cut down on the confusion, the loneliness and so on. You help each other find a positive way to express yourselves and focus your energies. If either of you is floundering in any way in your personal life, this relationship helps you calm down, gain focus and find a direction.