Some sort of struggle between you, subtle or overt, will pit you against your friend's path in life. Somehow, you two have needs or aims that will be at odds with one another. You could start off feeling perfectly in tandem -- trusting of one another, sharing goals and ideals.
But later on, it will become more apparent that you're following two very different paths, ones that may even work at cross-purposes to each other. You'll find that you have very different views of the world or different ways of approaching challenges. It's all a matter of perception: What looks like weakness is simply a different personality from your own, and what feels frustrating about your friend's life choices is actually a frame of reference for what you don't want to do in your own life. You could stand to learn from one another if you work to maintain this friendship.