Woody Allen once said that relationships are like sharks, they die if they stop moving. Accordingly, many people unquestionably feel a need to get closer, move in, and get married, etc., because of a belief that they will be happier.
In my experience, the belief is more common in women who anticipate a sense of security but who paradoxically may become more apathetic to the male’s needs which results in considerable “first year of marriage” disillusionment. This is kind of like the erroneous belief that having kids increase marital harmony (which it generally does not).
One thing I do hear consistently is that threats and guns to the males’ head to elicit a commitment do not inspire feelings of love. Most men want to be honorable and will eventually belly up to the bar.