It's often related to their feeling that attractive and compatible women are a "scarce resource" that must be mined to exhaustion, rather than that they need to expand their search, and perhaps change their own perceptions of who might make an appropriate romantic partner.
Another common cause is even simpler: there are many "additively attracted" men – those who, once drawn to someone romantically, will never lose that feeling (at least until it is acted upon/consummated).
Even if they later happily marry or become involved with someone else, the feeling may never diminish in intensity, though it may become less urgent or otherwise easier to suppress.
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