Article here. Excerpt:

'People often speculate they must fear commitment, have been betrayed in a prior relationship, or have some type of personality disorder. Research indicates, however, that the explanation may be far more straightforward.
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We tend to psychoanalyze relationship-reluctant individuals as if something was “wrong” with them. What happened to them in the past? Were they abandoned in childhood? Did an ex-partner betray them? We also stereotype men as more likely to be gun-shy regarding relational intimacy or walking down the aisle.

Sometimes, relationship reluctance is indeed due to a past betrayal or broken heart. Other times, however, it is simply due to personal preference. According to research, for self-assured men, in particular, singlehood signals success and satisfaction.'