I think Ari has shared some valuable insight already - pointing to insecurities.
Different people seem to put different parts of a relationship on a higher rung. Problems don't occur until someone starts sawing at the top rung. I guess they think that's all they have left to cling to before falling.
The security thing Ari mentions.
And of course there's a whole host of things that contribute to people's insecurities. We all have them. But - at least for me - I find it fun to have one of them exposed. That way I can work at reinforcing it. I don't want any 'top rungs'. I'd rather weave my big net on the way up so regardless of which rung breaks, I'll land somewhat safely and start climbing again.