The subject is, far and away, the absolute most important consideration in making a photograph. Period. While there may be other considerations, the subject makes or breaks a photograph right from the start.
It’s what the photograph is about.
Pure and simple.
It draws the reader's attention. It establishes a connection with the reader. It establishes commonality with the reader. It is this commonality that elicits an emotional response rather than an intellectual one and allows the reader to relate to the work experientially.
There are some requirements, however.
The subject must be tangible. That is, it has to reflect light. While your topic may really be the relationships between people, it is the people in the photographs who are demonstrating those relationships. We see it in their faces, their body language, etc. So, in reality, the subjects here are the people.
The subject must be readily or easily identified. It doesn't have to be the largest thing in the frame, but it has to be easily identified as the subject. It is the "what" in "what the photograph is about." It has to be clear, concise and coherent.
Because photographic subjects exist in the real world, placing them in a real context is critically important so that sense of commonality with the viewer can be developed. A portrait has a totally different context if done in a sterile studio rather than in the subject's natural or normal environment. The context of the photograph gives more information about the subject and a better idea of what that subject is all about.
It bears repeating that in presenting the subject, the photographer translates all of his or her perceptions, ideas, impressions and understandings of the subject into the project and, therefore, into each photograph in the project.
Let's say there are two photographers... one sees the automobile as a necessary evil and the other sees it as a liberating device. The first may show the city streets, choked with cars lined up bumper to bumper not really moving and people dodging between them. If the light is right, the photographer may include the clouds of exhaust fumes rising above the gridlock. The second may show the automobile cruising along an open road that is free of congestion, convertible top down, under a sweeping, bright blue sky. Both are photographing the automobile, but from drastically different perceptions, impressions and understandings.