The notion that you are going to find the perfect person for the rest of your life at any given moment in time is ridiculous - you are going to change, they are going to change, your relationship is going to change both of you. The sense of friendship, being at home with one another, and the mindset that a marriage takes work have all been instrumental in my own marriage so far.
My wife and I met in our early 20s. The things we wanted out of life - both individually and as a couple - have changed significantly over the past decade. It is likely that if either of us had known where we would end up we might not have chosen our past selves to be together with our future selves. However, we've been a part of every individual and collective change and have worked to build a strong relationship through the changes. There is something amazing about walking through adulthood together and staying committed through all of the ups and downs.