Healthy relationships are dependent on the health of the people in the relationship. This is not exclusive to physical health; in fact the physical health is lower in importance for realizing a healthy relationship. Mental and spiritual health are the most influencing factors when it comes to the health of a relationship.
The easiest way to explain mental health is to say it is how an individual views and treats themselves. Yes, mental health is often influenced by outside stimuli, but it still boils down to how the individual responds to that stimulus towards themselves. Besides, life in itself is nothing more than an accumulation of outside influences and experiences. It is how that individual responds, grows and matures from those experiences that measure their mental health. By looking at it in that manner, it is much easier to see how important mental health is to a healthy relationship. If someone does not have a healthy relationship with themselves, then they will not be able to experience or contribute to a healthy relationship with anyone else.
While typically people tend to associate spirituality with religion, there is actually a great difference between the two. For the concept of healthy relationships, we will have to separate the two even more. Spiritual health is better associated on how an individual views and treats others. This is our spiritual health because, if we are kind and generally loving to others, we tend to be spiritually sound. But if someone generally takes advantage of others and generally mistreats people, then their spiritual health is very lacking, and over time can completely dissipate. Spiritual health may be more apparent to the contribution of a healthy relationship, as it is how someone treats others. Since a relationship is the cohesive interaction and growth of two individuals, each individual's spiritual health is important.
The interesting thing about the spiritual health is that people who have a poor spiritual health will typically find themselves partnered with those who have poor mental health. This type of coupling tends to produce not only an unhealthy relationship, but more likely a completely toxic relationship. When people get depressed, place their personal happiness on the shoulders of someone else, they open themselves to those who thrive on that negative energy. It is this type of relationship that is most common, and of course the most damaging to both individuals. People with sound mental and spiritual health might find themselves with someone who is lacking in either area, but they will remove themselves from this unhealthy relationship as soon as possible. Failure to do so, will begin to drain on their own health, until they eventually, for lack of a better word, compliment their partner deficiency.
Understanding these basic foundational concepts will provide you with the pathway to be able to contribute and experience healthy relationships. Not only healthy romantic partnerships, but friendships and even family as well. If you find yourself in unhealthy relationships from time to time, then that might be a good indicator for you to look at yourself. Now if you find yourself in those toxic relationships, especially if they seem to go on and on for long periods of time, then you must immediately look at yourself. Focus on your own mental health. If you are unable to look at yourself alone, then get some help. Work on the things you don't like about yourself, retrain the way you treat yourself. Focusing here is where your efforts will pay off the most, as an individual's mental health will affect their spiritual health. Raise one, and the other will follow. Sometimes your mental health will depend on your physical health. If so, then start there.
The most important thing to remember is that if you want to have a healthy relationship, you have to be healthy yourself. You cannot depend on someone else to do this for you. It is completely impossible. Healthy people will not stay with unhealthy ones. And if you are able to bring a healthy person to an unhealthy level, then it is no longer capable of being a healthy relationship. So fix your broken parts till you can smile at yourself in the mirror. Treat others as you wish to be treated. Respect yourself, and don't allow others to take advantage of you. Be willing to go out of your way for someone special for nothing in return. When you are able to be at that point, you will find yourself with someone similar to you, and the two of you will experience the most incredible, lasting, healthy relationship you ever imagined.