The determination that we would not separate may play a big role. Once you have decided the goal - no separating, you will act towards achieving that goal. Please make this bond with your partner that you will not separate. I wish to add something here. When things are going smoothly, all such promises sound very good. As soon as a crack appears, the same looks very difficult. The need is to take care when the cracks appear.
Why do the cracks appear? Miscommunication, Discords, Different views on very important matters, career problems, health problems, third person entering the relationship, and many other factors can play role in this. As time passes, distances may grow. The reverse also happens. But now days it seems that the earlier case occurs more often.
Do you want to grow together till old age? Are you sure that your choice of partner is perfect for you? Is your partner also sure? Both of you have considered every factor? If there are any complaints, you are ready to sort them out? Do you keep the communication open? Do you love each other? What about caring? Please consider all these and draw plan so that separation becomes a very difficult possibility.