The friendship you'll share will actually hold your friend back from moving forward into the future that they need to embrace. Somehow, your friendship will be linked to your pasts; perhaps you'll find out that you knew each other long ago, or that you had friends in common, or perhaps it will just be that there's a familiarity between you -- a sense that you've known each other for a lot longer than you really have. But staying locked into old patterns of relating to each other will be detrimental to both of you.
It could cause the wheels of your friend's journey to come grinding to a halt, stuck in one position on the tracks. Moving forward will be difficult, because it'll be a challenge to the status quo within your friendship -- and the status quo, after all, is what the two of you will focus on most. Familiarity can become addictive.