But back to the 'ladder theory', because this one mutated and burrowed it's way into the popular culture and consciousness, even inspiring the aptly named MTV show 'the friendzone' in the early 2010's.

The one ladder leads to a sexual partnership relationship. The other _only has friendship_ on it. :blobcatsweat:
Hence these men would advice each other to watch carefull which ladder they were climbing. If they noticed themselves climbing the friendship ladder, it was pertinent they 'hop ladders', meaning doing something to get onto the sex/relationship ladder. Lest they get stuck on the friendship ladder, or in 'the friend zone'.

Small correction "the friendzone" was in 1994 already used and popularized in 'Friends' the TV show