why mostly girl friends do not marry with their lovers or boy friends ?

I'm not sure what you mean, but if I understand your question correctly, people's needs and desires change as they get older and more mature and are exposed to more people and more possibilities. Also, some couples are together for security more than romance, and then find someone else they are truly romantically (or physically) attracted to. Others can't make a commitment to stay with one person for one reason or another, such as a bad previous experience, and some are always selfishly watching for "something better" to come along (like girls who won't accept a prom date until the last minute because they really hope that Jimmy will ask them!).

And since you put this question in this particular category, some couples also are simply prevented from getting married, by their church or by law.