list is dominated by european nations - particularly those in scandinavia .
switzerland is now the happiest nation , with iceland the most rapid riser .
reasons range from strong gdp to beautiful scenery and a vibrant culture .
uk ranks at a lowly 21 , although brits are happier now than back in 2012 .
world 's least happy nations are unsurprisingly places ravaged by war and extreme poverty , such as syria , burundi and togo .