Why are men afraid of being alone?

You cannot generalize this as there are men and women that are "afraid" of being alone. It can be an emotional issue, traumatic issue, stemming from childhood, or lack of self-esteem/independence which cause a co-dependence need etc. - everyone has there own reasoning or personal explanation. Whatever the case may be they would need to first address the issue, possibly seek counselling if need be and take the time out of relationships on their own and gain the independence required.