From "On Truth" (Ch. III) by Princeton Prof. Emeritus of Philosophy, Harry G. Frankfort. (Little book next to the US Constitution by the checkout at Borders.)
Spinoza defines joy as "passion by which [one] passes to greater perfection." When our capacity [to reach goals, i.e., our development] is increased, we experience a sense of increased vitality. (Similar effect to exercise.) Expanding one's ability to realize and sustain one's true nature is inherently exhilarating.
If one recognizes that joy (stemming from the process of increasing in one's capacity) has an external cause, one inevitably loves that object or person. We can't help loving (protecting and nurturing) that which we recognize as a source of joy, that which helps us become ourselves.
People cannot help loving truth because we cannot help recognizing that truth is indispensable to enable us to stay alive and to live fully in accord with their own natures. If one is indifferent to truth, one is indifferent to or despises one's own life.
Normally, with people we don't know well, we assess whether what they say is true by reference to our own observations. Over time, we begin to rely on people who win our trust. If we believe them, liars damage our grasp of reality -- they intend to make us a little crazy. We realize that in believing them, our judgment was flawed --we lose confidence in our ability to discern truth, to decide who to believe, and we begin to question anything the liar said.
Spinoza [1632-1677] is best known for his "Ethics" -- a monumental work that presents an ethical vision unfolding out of a monistic metaphysics in which God is no longer the transcendent creator of the universe who rules it via providence, but Nature (understood as an infinite, necessary, and fully deterministic system of which humans are a part) is supreme. Humans find happiness only through a rational understanding of this system and their place within it.
Perhaps ironically, Spinoza, whose definition of love works for me, was never married, never had children and didn't even ever have a steady.
Professor Frankfort's clear perspective on the vital importance of truth and why we need and love it, gives me some hope that humans may begin to embrace the "inconvenient truth" that we are causing and can mitigate the damage from climate change. Our experience with life tells us that lies lead us into danger.