No one knew about their problems until then. Bonding should work to prevent problems, not create them. Of course, single people find it easier to bond, but if a new missionary expects bonding to get al his or her needs, it won't work. Even after making good friends with local people, great loneliness will ensue, because the understanding of the foreigner is bound to be limited. Singles face a small pool of potential close friends, so while they try to cope with their problems, they have few friends with whom to discuss them.
Married couples seem to have enough struggles of their own to pay attention to the singles.
If marriage does come hastily, without adequate appreciation of cultural differences, severe stress may result in years of unhappiness. Married missionaries can rely on each other and find strength, provided they have become good friends and companions previously. But if their relationship is rocky, friction will mount under the new pressures on the field and cause defeat. I knew a couple that expected their problems to vanish once they got to the field and became missionaries, I suspect because the believed that would make them more spiritual. Field stresses exacerbated their marital problems and they did not return after their first term.