I think some people who have long engagements start to forget why they're getting married and after all the planning and all the stress and the big party, they are tired and feel like they have nothing to look forward to...
Plus some people think the other person will be different when they get married and that doesn't usually happen... and sometimes you think the other person won't change at all, and that is unrealistic too! If he didn't take the trash out before you got married he's probably not going to start just because you are married.
And some people think they can relax now that they've found the right person, forgetting that a relationship is a living thing that constantly needs to grow, there needs to be love, respect, and communication no matter what.