You might have any number of reasons for this, but since I want a sacramental marriage (meaning that, like the other sacraments, it will bring me closer to God rather than pull me away from Him), I couldn't consider anyone who would be ineligible to receive the Sacrament of Matrimony.
Note: If you are married but doubt exists as to whether your marriage is a sacramental marriage or what is known as a "putative" marriage (meaning that you appeared to be married but really weren't) please ask your parish priest to look into the possibility that a declaration of nullity could be obtained for that marriage. In general, over 90% of requests for declarations of nullity are granted these days.