Never let emotions influence business decisions. You and the CEO may have a good relationship, but when push comes to shove, the cold hard logic of economics always comes first in business - on both sides. Be professional, say 'I've decided to take on a new opportunity, new challenges.." etc. Leaving the company in a lurch is only a positive thing - it will make them (hopefully) realize something is off with staffing or something else is not right with the business.

I have always tried to keep in mind the dictum that when you become essential, it is time to quit. Every time I've forgotten that, I have eventually suffered. It is a sign you are probably overworked, putting the job first for no reason (if you don't have equity) and probably aren't paid enough either.