Cohabitation can happen for any number of reasons. People may fear commitment or be unready to commit to one another. They may dislike the idea of traditional marriage because they think it is patriarchal or because they have had a bad marriage in the past. Or they may simply be cohabiting because they are in love and have planned to marry (perhaps even having a date set) but do not want to wait for that date to start living together.
There is some controversy over the effects of cohabitation. Some researchers believe that it shows a lack of commitment and therefore leads to weaker marriages more likely to end in divorce. Others feel that it makes for more stable marriages because it allows the partners to enter marriage with "their eyes open." My own view is that the impact that it has on marriage depends largely on the reason for the cohabitation. Couples who cohabit until the time is right to marry are probably strengthened while those who do so out of a fear of commitment are probably not.