Pothos is a kind of yearning or longing. Aristobulus, one of the biographers of Alexander, thinks that this may have explained his drive for foreign travel and exploration.
It was produced by Ptolemy I within a few years of Alexander’s death, so has this in its favour. However, it may have been part of the subsequent mythologizing of Alexander.
Sources suggest that they were exhausted by the Indian campaign and the fierce fighting there. They had been away from home for years and were undoubtedly homesick and exhausted. So they refused to go on.