Once you have established a relationship that you value, why would you ignore it allowing it to dissolve? Most of us wouldn't. On the other hand, most of us haven't looked at what it takes to maintain a relationship in the first place.
First, let's look at what it takes to build the relationship since that is what we hope to maintain, in fact build upon. Imagine a couple, they see each other and they notice a wide range of things: activities, goals, personal awareness as well as other awareness. We like all those things. They apparently feel the same way about you. Fact is, both of you spend a great deal of time together and find out about one another. Put another way, each allows the other to build a representation of themselves in their mind. This is the person that they will carry with themselves over their lifetime. Of course it will need to be updated and revised since each of us is a work in progress and subject to constant change.
Second, there is never a time when you aren't building a “you” in their mind. (You cannot, not communicate.) Since the building process is time and energy consuming you are either building a stronger and better relationship or their “image of you” is fading (dying). If it took time and energy to build the relationship you will have to spend time and energy to continue the process. Somehow, we have come to believe that once established it is like a sturdy building and will be there forever. That will eventually lead to fights, affairs, diverging interests and divorce.
So what is the most likely answers to this problem? 1) Recognition that life and relationships are processes and the process is only finished when one of you dies. 2) If you hope to build on similar interests and goals you will have to spend the time and energy necessary to pursue interests together, try new and different things together, and continue to repeat the shared interests that brought you two together in the first place. 3) If you are spending more time with a professional colleague than you are with your significant other, then it is very likely that the person and environment that you're building in your mind will be in conflict with the world you built with your significant other. It just may be true that “When you're away from the one you love, you love the one you're near.”
An excellent reason for early and continual discussions about personal and professional goals and how they should, or should not be pursued. An excellent reason for selecting a profession that will allow you to maintain what you have so painstakingly created over an extended period of time. An excellent reason to recognize that it may not really be possible to do anything to which you set your mind.