In Ideas Have Consequences, Richard Weaver talks about the relation of reason to sentiment (not "sentimentality"): "We do not undertake to reason about anything until we have been drawn to it by an affective interest" -- in other words, we must care about something before we will bother to think carefully about it. He then goes on to say, "We have no authority to argue anything of a social or political nature unless we have shown by our primary volition that we approve some aspects of the existing world. [. . .] We begin our other affirmations after a categorical statement that life and the world are to be valued. It would appear, then, that culture is originally a matter of yea-saying."
I like this. It is so easy to get caught up in "what's wrong with the world" -- after all, there is a great deal wrong with it! But if we forget or refuse to acknowledge that there is good, that there is that which should be valued, then we argue from negativism, bitterly and hopelessly. The best art affirms even as it critiques; that which can be affirmed forms the standard by which we critique that which has fallen from it.
But it's not mere sentiment that's needed, not sentimentality, by any means. Sentiment, to provide the foundation for right reasoning, must be formed and informed by what Weaver calls "the metaphysical dream" -- an understanding of design and purpose outside of and beyond man himself: "There must be a source of clarification, of arrangement and hierarchy, which will provide grounds for the employment of the rational faculty" (and which forms the sentiment).
How is this "dream" inculcated in us? The "poetry of representation ['mythology' broadly understood as answering our questions about who we are and how and why we are here], depicting an ideal world, is a great cohesive force, binding whole peoples to the acceptance of a design and fusing their imaginative life. Afterward comes the philosopher, who points out the necessary connection between phenomena, yet who may, at the other end, leave the pedestrian level to talk about final destination." Both art and philosophy are needed, then -- art to show us the ideal, to move us, to make the dream alive to us; and philosophy to help us understand how the metaphysical (including the metanarrative of art) and the physical world are conjoined, to be able to articulate the dream when necessary, to reason from it to evaluate action.
He sums it up like this: "Thus, in the reality of his existence, man is impelled from behind by the life-affirming sentiment [belief that there is that which is of value in the world] and drawn forward by some conception of what he should be [pictured by the artist and articulated by the philosopher]."