Committing yourself to someone else for the rest of your life requires you to let go and trust them. You have to trust that they'll be faithful to you in every way, that they'll always be there, and that your love for each other will be able to carry you through any disagreements that you have over the years. Most of us are willing to make the leap once we find the right person, because we know that there are so many wonderful things that come with making that commitment.
Unfortunately, not all men are able to let go and take that leap of faith, especially if they've been burned before. They worry incessantly about whether their fiancee really loves them and wants to be with them, whether she's going to cheat or if she's been honest about everything. It often comes from his insecurities. And when trust issues are deep-seated, sometimes no amount of reassurance from the woman can make them go away. While counseling can help, it might be something that the concerned groom has to address on his own.