It is now well documented that social relationships have an influence on mental and physical health. Consequently, if you fear developing low self-esteem, depression, schizophrenia, eating disorders, drug or alcohol dependence; if you fear tuberculosis, coronary-associated diseases, cancer, immune or endocrine-related health decrements; if you fear committing suicide, being the victim of a homicide or an accident; then you may want to consider that (a) very rarely has it been observed that all these problems intervene at the same time in one's life, (b) being embedded in social structures lessens the risk of morbidity as well as mortality, (c) the availability of emotional support, i.e., having someone to confide in, is a key factor for good health.
Thus, you may surround yourself with friends, lovers, and relatives, or develop new group affiliations and enhance the frequency of your contacts. Still, we must not lose sight of how these results were obtained.
Epidemiological, longitudinal studies consist of a follow-up of large groups of individuals that differ along one criteria - typically, marital status. In studies monitoring physical health at the outset of the study along with factors such as frequency of physical activity, cigarette smoking or alcohol consumption, married people appear to live longer than those who are not married, and this effect has consistently been found to be stronger for men than for women.
Nonetheless there are also studies addressing the possible effects of social support - and notably, marital support - on physiological indicators such as cardiovascular, endocrine or immune reactions. It has been found that social support tends to buffer the morbid outcomes of stress. But what happens if the presence of an intimate partner is in itself stressful? What happens if conflict or escalation of negative behaviors are the norm for certain couples?
Marriage provides health protection, but marital conflict and dissatisfaction may be detrimental to health. Emotional support may turn out to be an "emotional stressor", thus invalidating a positive outcome regarding health. An answer to the question posed in the title could be that, as long as one is not dead, widowed, or divorced, the positive effects of marital interaction overcome the incidence of negative effects such as disappointment, sentimental wear and tear, or personality incompatibility.
In any case, it is not marital status per se that protects us, but the presence of a loved one, someone whom we think loves and cares for us. It is the idea of love that offers us protection. Further research might aim at testing the idea that couples tend to last as long as the idea of love is maintained between partners.