Be honest. Be polite. That's it. I quit quite some jobs so far and know the "But the guy's nice" attitude far too good. That doesn't matter here. You're ready

Be honest. Be polite. That's it. I quit quite some jobs so far and know the "But the guy's nice" attitude far too good. That doesn't matter here. You're ready to move on and romanticising (is that even a word?) your connection to the boss and trying to spin analogies to splitting up a romantic relationship are not useful. You're a professional. Your boss is. He's seen other people leave - and it's again just a matter of business, nothing personal. Just remember that you follow the rules in the first sentence. If you explain yourself honestly and do it in a polite way, you've probably a spot to return to, if you should want to in the future..