And so do some women. This isn't gender-specific.
Some people do experience fear when falling in love—because it's really scary to be that vulnerable! When you're falling in love, you have the possibility of getting hurt. Some people instinctively run away from serious relationships because they're too afraid of that possibility of heartbreak. Again, it's easier to choose to leave yourself than to suffer through a possible rejection.
This behavior is common among people with an avoidant attachment style. Your attachment style is your way of behaving in relationships, and it's shaped based on your earliest interactions with your first caregivers. The three main adult attachment styles are secure attachment (you can easily love and be loved by others), anxious attachment style (you tend to need a lot of attention and validation to feel love), and avoidant attachment style (you tend to need a lot of space and can feel suffocated in relationships). Some research suggests men are more likely to develop an avoidant attachment style, potentially because of the aforementioned differences in how boys and girls are treated in childhood.
"Generally speaking, I don't think men are afraid of commitment," Kahn adds. "One could argue that because masculinity can be associated with not being vulnerable, and commitment and intimacy involve vulnerability, that some men may appear or experience fear or resistance to commitment."