The power dynamic between you may be shifting during this period, or it could be that as a result of getting more deeply involved in this relationship, the foundations of your own lives are changing. Perhaps you'll go from dating to living together during this two-year transit, or from living together to a formal, long-term commitment. These graduations are supposed to be good things (and they are), but they can come along with a certain sense of melancholy as you realize that every gain comes hand-in-hand with a loss.
You may lose touch with old friends who no longer fit into your lifestyle, or the transformations of this period could occur on a more personal, deep level. Perhaps being in this relationship means you have to give up parts of yourselves -- old, habitual behaviors or fantasies of some idealized love that will never come true. If these shifts make you feel sad or vulnerable, talk them out together, and give support to your sweetie. And remember that life is all about change, and growth, and progress from one thing to another. With every loss comes a gain; every ending marks a new beginning.