In this film, Mr Big got cold feet after Miranda told him that he shouldn't get married. Usually he realises that he can't make a commitment to his fiance because he feels too young, inadequate, is immature or just doesn't love her enough etc.

yes, hard for me to understand this also.. for me, marriage is important, if you are going to marry sb, you never abandon them. If what you describe happens, I think the bridegroom got scared, realized he didnt really love the girl and wanna be with her forever, its sad really, but some people make promises they cant keep, and he should not have asked her to marry!