What's funny is how it relates to the Perception of Power. No one wants to submit to someone who's presumably of equal power, but if a "King" comes along, its an easier pill to swallow, swearing fealty.
When you are a Count and want to become a Duke, you need to ask yourself which is the easier path: to force other counts to submit to you, through strength and conquest that you will have to prove alone, or by submitting to a Duke, help him become King through strength and conquest as part of a an already formed grouping of skilled and rich nobles, and hope he makes you Duke when he becomes King? Both historically and in gameplay, I think the second route is often the most sensible one.
