If you go through life living out comfortable routines, that gets shaken up in this relationship. The way you usually do things flies out the window, and you're called on to change along with the relationship. Even if you both try your hardest to keep things smooth and seemingly 'normal,' things have a way of spinning out of control of their own accord.
You go through big changes and dramatic emotional displays, while your partner is busy trying to resist the changes and hold onto the status quo... After a while, it becomes more and more obvious that the two are mutually exclusive. Neither of you is able to hold on to the way things are while progressing together as a couple, or as individuals. You both have to dig deep and figure out where the incompatibility is occurring. Do you have similar styles sexually, or are you always shocking your partner with your sense of freedom and abandon? Are you pursuing a career that seems unstable or even unseemly to your lover? Or do you just have a much more varied and transformative emotional life than they do? Wherever the differences lie, if you can look at them honestly together, you're able to find out how to learn from each other and grow together.