Being in a relationship is a like having a contract. Two people agree to form a partnership, a kind of Corporation called "You & Me, Inc." They pool their resources and agree to abide by certain bylaws. Some of these bylaws are recited at a ceremony: to have and to hold, for better for worse, for richer for poorer, in sickness and in health...blah, blah, blah. Others are unwritten, but universally accepted, as a part of the relationship contract whether the partners marry or not: no cheating, be honest, thou shalt carry thy weight in the relationship, etc.
Any long lasting relationship will require that the partners rewrite the terms of the agreement from time to time. Why? Change. People change. Expectations change. What they want out of a relationship, or what they are willing to give to another person, can change. When this happens, it's not the death knell of the relationship. However, both partners must be willing to come to new terms in the agreement if the relationship is to continue.
The changes can generally come about in three different ways: (1) Something more, (2) Something less, and (3) Something different.
One of the partners may develop more expectations of their life. For example, they may want to go back to school and this was never discussed or even anticipated when the couple married. That couple will now need to rewrite their contract and come to a new agreement if they wish to continue the relationship. Who will support the family while the other person is in school? Will this change put off having children? If there are children, how will childcare be arranged? How will the education be paid for? There are a lot of terms to negotiate.
Another type of change is made when a partner wants less of something. A partner may want a less stressful job. This may also translate into less money, in which case, the partners will need to come to new terms of the agreement. How will this effect the budget? A common re-written relationship agreement involves a parent wishing to quit work to stay home with the kids. This will suddenly translate into a one income family, something one of the partners may have never anticipated.