There is a whole world of difference between enthusiastic, blind optimism and a safer, prudent attitude based on reality.
Crossing your fingers and hoping for the best after jumping off a cliff is no way to look after the population you govern. It's ok to risk your own individual future, I suppose, but not the futures of a whole country.
The reality is that our record of completing deals has been abysmal recently and now way too risky. We all know that the world cannot trust Trump's word and Boris pushes him close in that respect! He is more likely to be laughed at rather than respected as a serious politician.
We have suffered a succession of Prime Ministers who have made catastrophic decisions in terms of the welfare of the vast majority of ordinary citizens. What evidence is there from Boris' past record that makes you think that he will safeguard our interests in any way? He has proved himself a liar, an egotist, a misogynist, a bully, a thug, a racist, and an empty vessel whose gaffes and lack of diplomacy have proved disastrous in the past. There are numerous facts (too many to list here) that confirm these traits.
Cheery optimism is refreshing after the austerity policies of the Nasty Party and the tiresome lies and struggles of the Brexit negotiations, to the point where we WANT to believe that behind the inarticulate bluster and arm-waving there is something more concrete to believe in.
All the evidence tells us that there is not.
He was elected, as May was, by Tory members only, so the whole country has not spoken even though facts have arisen since the 2016 referendum that undoubtedly have changed many people's minds about the outcome of the vote. Democracy entails letting the people speak with up-to-date information at their disposal rather than rely on a decision from three whole years ago that is now discredited, just as the proponent of that referendum has been.