For me, it's pretty simple really and a recent trip on the monkey to Wales was it. Sun shining, a couple of nights away, just riding some fantastic mountain roads at a leisurely pace, stop/start photos, no people to piss me off, just the odd natter to like minded folk at parking spots.
Also, sitting outside a pub on a sunny weekend break with Mrs D just people watching, or whatever else takes my eye.
, along with walking by the sea. Simple things really, but you know where you're happiest.
Best when on my own 
I was going to post this as an example of places that I remember with a degree of euphoria.