Responding to NBS's post on 'Soulmates', we often have the misconception that in marriage the equation works out as 1/2 + 1/2 = 1, by which we assume that we are incomplete until another completes us. That's not going to happen. A person who only feels like 1/2 and seeks an identity in another is going to be disappointed that the perfect "1" will always seem elusive, unattainable even.
Marriage is more of a 1 + 1 = 2 situation -- a partnership of 2 complete individuals who are different from each other, but preferably in a complementary way. Because marriage comprises 2 complete individuals, staying together is a constant choice they must make with every obstacle or difficulty they encounter as a couple.
Kids add a further complication. They cannot be expected to be complete in themselves until they leave to build their own nests. So, 1 + 1 + x/2 = 2 + x/2. Chaos ensues.