How do I not feel guilty leaving a good friends company?

I work for what has become a close friends company. He and myself are the only employees and I am essentially his apprentice. I already left once before, and came back and he welcomed me with open arms.

I now have a better opportunity I am thinking of leaving again for. How can I move past the guilt of leaving? I feel like this will really surprise him and probably crush him as he thought I was going to take over the company one day. Any help is greatly appreciated!