In the long run, to me what has been the most important thing is a willingness to allow change. Life is a journey with many unknowns. The best traveling partner is one who who will forgive the bumps in the road and accept that sometimes they happen. Be able to listen, and respect the other person's decisions, even though they may not completely understand. If something happens, and you break down, that person will help to fix the problem, in order to continue your journey together.
They will laugh at your stupid jokes, listen to your crazy ideas, and gently steer you back, when you've gone too far astray.
They know that one day the journey will end... So throughout your travels, a good partner will always in some way, remind you of the wonderful things you both have seen and experienced together.