Think of it perhaps in simple terms of pecking orders.
There will be people whose lead you just naturally "follow" and there will be people who just naturally "follow" your lead. No change of character on your part, no change of attitude on your part. Only a change of "duties". You do it all the time at work and within your familial settings. Probably with friends as well.
So when it comes to dominant and submissive tendancies, the same it true. The biggest difference between doms and subs, versus switches, (from my perspective,) is that when a switch meets someone who is more sexually dominant or submissive, they are willing to change their own role, their duties within a relationship.
Whereas if I happen to meet someone "more" dominant whom I find sexually attractive, I am more likely, make that far far far more likely, to suggest sharing a sub than I am to suggest I would submit to them sexually.