Sheri Berman has an excellent article up on the Social Europe website (a very useful site incidentally for coverage of trends and debates within European left politics). In the article she explains how important optimism has been--and should be--to the left.
"The rise of right-wing populism is probably the most pressing problem facing Europe today. Many analysts, including myself, have linked the rise of populism to the decline of the social democratic or centre left. Many traditional social democratic voters now vote populist; social democracy’s embrace of a “kinder, gentler” neoliberalism opened a policy “space” populists filled with welfare-state chauvinism; and social democracy’s fading electoral fortunes have rendered majority left government and, in many European countries, any stable majority government impossible, making it more difficult to solve problems, increasing dissatisfaction with democracy and support for populism further.
But beyond these connections lies a more fundamental one: the loss of a sense of the possible social democracy injected into post-war liberal democracy.
Social democracy was the most idealistic, optimistic ideology of the modern era. In contrast to liberals who believed “rule by the masses” would lead to the end of private property, tyranny of the majority and other horrors and thus favored limiting the reach of democratic politics, and communists who argued a better world could only emerge with the destruction of capitalism and “bourgeois” democracy, social democrats insisted on democracy’s immense transformative and progressive power: it could maximize capitalism’s upsides, minimize its downsides and create more prosperous and just societies....
Populism peddles a politics of fear—of crime, terrorism, unemployment, economic decline, the loss of national values and tradition—and asserts that other parties are leading their countries to disaster. Surveys make clear that populist voters are extremely pessimistic: they believe the past was better than the present and are extremely anxious about the future. But pessimism has infected Western societies more generally. A recent Pew survey for example revealed that even though growing percentages of European citizens view their country’s economic situation as dramatically better than a decade ago, this has not translated into greater optimism about the future. Indeed, in many European countries the “experience-expectation” differential has grown: in the Netherlands, Sweden and Germany, for example, approximately 80 percent or more say the economy is doing well, but less than 40 percent believe the next generation will be better off than their parents. These views reflect a troubling reality: particularly in times of change and uncertainty, people’s views are shaped more by emotions than rationality. Recognizing this, Roosevelt, the SAP [Swedish social democratic party] and earlier social democrats understood that for the center-left and democracy more generally to thrive, what was needed was not merely practical solutions to contemporary problems, but also an optimistic vision to counter the dystopian one offered by populists.
During the postwar decades social democracy provided just this. Against communism and liberalism they argued that people working together could use the democratic state to make the world a better place. The problems of the 21st century are different in form, but they are not different in kind. What is needed is a combination of pragmatic policies that can address challenges like economic inequality, slow growth and disconcerting social and cultural change as well as an ability to convince citizens that liberal democracy provides the most promising path to a better future. The rise of politicians as different as Trump, Corbyn and Macron makes clear how desperate many citizens are for leaders who insist that politics matters—that change is possible if the will is there."
In other words, Berman is saying that an optimistic vision isn't just an option for the left--it's a necessity if the left is to thrive. I could not agree more strongly. In fact, I wrote a whole book based on this idea: The Optimistic Leftist. Please consult it if want to see this argument developed at greater length.