There are several factors that can affect the strength of a relationship. It does not necessarily only apply to romantic relationships; family/friend bonds count as well. They could be internal factors, which are the problems that lie within the people involved. They could also be external factors, which are problems from external sources.
Okay, I initially typed out a whole chunk of words but I deleted them because I deemed it nonsensical and full of rubbish.
Back to the question, in my opinion, communication can be considered the "unit" of strength of a relationship. That means that the inability to communicate efficiently will eventually cause the downfall of a relationship. Communication can be affected by both internal and external factors.
Clear communication is crucial in any relationship. Messages relayed by verbal or non-verbal cues can be misinterpreted by the other party, leading to unwanted misunderstandings. If your loved one(s) is unable to decode the message you are putting across, the reply that you receive would be flawed as well. You have to be transparent with your loved one, "educating" them with what you feel and think so that they will have a better grasp of your "style" of communication; let them see the world through your eyes. Communication needs to be constant, if not contact could be eventually lost. A mutual understanding is required for a relationship to grow or be maintained. Once this mutual contribution is absent, there will definitely be pain for at least one, if not both, parties. Once this pain is "unbearable", the relationship breaks down because we get tired.
We do not want to waste our cognitive resources trying to salvage something that will only bring us more pain; it would be like getting revived by the doctors just to go home and hang yourself again.

I feel like my mind is getting off-topic.