The male and female stay together for life. In the animal world, monogamy is quite unusual. Most male and female animals come together only for breeding. As you watch the gibbons you might also see them grooming each other, looking through each other's hair. They're not looking for bugs, the gibbons are actually quite clean.
Grooming is a social activity.
They do this to show friendliness toward each other. Grooming shows contentedness.