Below are some of the reasons a guy would date a girl for years but does not seem to want marriage.
1.He has decided early on that he has no interest in getting married to anyone at all. If so he should at least have the courtesy to let his girlfriend know at some point what his intentions are. Leading her on this way is underhanded and wrong.
2.He may just think it is more convenient to stay with a particular girl as long as he can with no intentions of marriage. It would be like taking advantage of her, she is with this guy when she could be with someone who cares.
3.His excuse may be he is waiting for his girlfriend to lose some weight, get a better job, or any other excuse. He could be waiting for his ideal woman that does not exist so he keeps making excuses. No matter what his girlfriend does it will never be enough, he will never propose.
4.Many times the guy says things like he wants to finish school or get a better job. This may be true and he may really want to get married. But if he drags it on for years then you may want to look for someone else.
These are just some of the more common excuses a woman may encounter while waiting for their boyfriend to propose marriage. There are more and hopefully you do not have to deal with any of these.