Generalization caveat: Not all men are afraid of relationships, but many men are terrified of them. Before I get into the reasons why they're so afraid, let me first address the question of whether men are more afraid of relationships than women.
The debate about whether men and women are extremely similar or extremely different doesn't seem to go away, and it's largely because we have little way of proving much within the psychological arena. Who knows, maybe one day we'll learn so much about the brain that we can definitively answer the question. Odds are, however, that the day may never come: Perhaps the social influences shaping males and females are so powerful that it's primarily the social part, and not the biological part, that makes men and women who are they are.
Do men fear relationships more than women? The truth is that it's hard to tell. Measuring fear of intimacy among men and women in a research sense is tricky, but one study (Thelen et al., 2000) attempted it and found that men scored higher on a Fear-of-Intimacy Scale. To women who have known men terrified of relationships, this research will come as no surprise.
Anecodotally, my fifteen years as a therapist have shown me that men are often more afraid of letting their guards down and being vulnerable than women, so it would make sense if they fear relationships more than women. To give some context, the media is always reporting about the different ways boys and girls are socialized, and many of us see such gender-restrictive parenting among folks within our social circles. Because it does appear that boys and girls, at least historically, have been socialized differently, it would make sense that girls who were socialized to engage in cooperative play grow up to be women who are better at handling emotions and relationships than boys who were socialized to engage in competitive and physical play and grow up to be men who are less comfortable with vulnerability and emotional intimacy in relationships.
But here's the important part: Not all men are terrified of relationships! When it comes to the subset of men who are, what makes them different? In other words, why are the so afraid of relationships?
Previous Relationship Trauma
A man may not be able to function well in a relationship if he has extensive issues that stem from a previous relationship trauma. The relationship trauma may have occurred when the man was a child or when he was an adult.
Men who, as children, had an absent parent, a parent they lost, or a parent who abused them in any way are going to have an awfully difficult time seeking out and maintaining a healthy relationship. The wake of trauma can make romantic relationships almost unbearable and undoable if the man has not processed the trauma and worked through all the associated thoughts and feelings.
In addition, men who are afraid of relationships may have had a previous relationship as an adult that was traumatic. Having a previous partner who abused them in any way, cheated on them, left them or died can cause these men later to avoid emotional intimacy and relationships altogether. Though some or all of these men may still have a desire for closeness, the emotional pain from the previous trauma is too great for these men to take the risk and jump into a relationship again.
OCD, Paranoid Features or Depressed Features
Some men are afraid of relationships because they have an overall approach that makes relationships exremely anxiety-provoking. Men who have OCD or OCD features are often afraid of relationships because of the uncertainty and lack of control that come with relationships. People with OCD have a very high need structure and need to feel that their environment is extremely controlled and predictable. The thought of dealing with messy emotions and having to share an emotional life, as well as a physical space, is often too much to bear.
Men who have a paranoid personality type are often afraid of relationships, as well. Paranoid men are hypervigilant about their environment and screening everyone who comes into their space. They are extremely aware of hierarchies of power and carry the ongoing fear that someone is going to trap them somehow and take advantage of them. For these reasons, reliance and dependence on another person are incredibly far-out notions for the paranoid man.
Suffering from depression or having depressive symptoms has everything to do with relationship status. For single depressed men, they suffer from low motivation and often feel badly about themselves. The idea of a relationship sounds extremely complicated to these men and, in addition, sounds like too much work for someone who is truly dealing with onging depressed feelings. Keep in mind that many men don't talk openly about feeling depressed, so you may not even know the real reason why a given man is afraid of relationships - you just know something is off.
If you're not a therapist (or a follower of any number of off-the-wall, extreme docu-reality shows), you would probably be more than a little surprised to understand how many addictions people suffer from. Some addictions are more obvious, while others are easier to hide. Part of what comes with the addictive process is extreme guardedness: The addict becomes hypervigilant about who they get close to and they avoid anyone who is going to hold them accountable. Think about it: Having a partner is going to cause an addict - someone in the throes of denial - to feel incredibly anxious, and the addict would find a way in such a situation to escape.
Now that you know about some of the reasons why men fear relationships, consider for a moment the paradox that many men who have a secret fear of relationships are often in relationships! In the relationships, these men often have a hard time maintaining closeness with their partner for any significant length of time: He either never lets them fully in, always keeping them at a distance or cheats, abuses, or witholds sex or affection from time to time. Other men who are afraid of relationships never even try to settle down: They're the bachelors at 40, never having married; the charming uncle who never brings the same woman to gatherings more than once; or the man who says he wants something long-term but distracts himself with types who are completely in appropriate, so the relationship never has any real chance of going anywhere.
If you have become involved with a man you believe has a fear of relationships, talk to him about it. Tell him what you believe and what you see, and do it in a casual, nonjudmental manner. If you really want to make a romantic relationship with him work well, offer to go to couples therapy to help him - and you, too! - flesh out these issues. There is hope for men who are afraid of relationships, but they must be disciplined about trying to change and honest with themselves about how dysfunctional their romantic life has been as a result of their relationship fears.
Thelen, M.; Vander Wal, J.; Thomas, A.; Harmon, R. Gender Differences Among Dating Couples. Behavior Modification, April 2000, 24 (2), 223-240.