as Jack is to-day: explain this How is "man only is interesting to man" quote is related to the viking ship experience? I mean, why is this said as conclusion to that experience?

The Vikings have been described as people who set out on sea voyages to hunt, raid, explore, and trade. The statement 'man only is interesting to man' refers to fact that the Vikings were interested in exploring far and wide so that they could find environments conducive to their survival.