Because Leopold Bloom doesn't lie. He doesn't pretend he's Living Your Best Life Now!  He's not self-actualized. He's not working on a glossy book celebrating The Spirit of Family .
People all over the world celebrate Leopold Bloom because he's trying, day and night, to see clearly, to move through what Joseph O'Connor  calls "the confetti-storms of memory in which most lives are lived." He's like our Gatsby:
So we beat on, boats against the current, borne back ceaselessly into the past.
The current, the storm, of our own story -- Many writers tell us about our story, but really only Joyce makes us feel on our flesh, moment by moment, the specific, encompassing force of its maelstrom.
Lots of novels are little more than variants on Freudian case histories. Ulysses is real individual life itself, in real time; Leopold Bloom's uncensored thoughts over twenty-four hours show us what consciousness feels like, how it operates. We are grateful to Bloom, we celebrate him on Bloomsday, because he embodies both the power of our personal currents, the way consciousness is beaten back by memory, and the heroic effort not to be dead-calmed by the past.
Unlike Jay Gatsby, whose consciousness remains mysterious to us, and who we know only obliquely, through a baffled narrator, Leopold Bloom is fully known to us. His stream of consciousness - unimpeded by any narrator - dominates the novel and gradually reveals the truth not merely of his life, but of our own.
Depressed by the contrast between his passionate youth and the onset of middle age, marooned in memories of his father's suicide and his son's early death, enraged and humiliated by his wife's infidelity, snubbed by his fellow Dubliners, Bloom, Homeric hero, struggles on against these storms. He keeps his bark afloat, and even manages to save Stephen Dedalus from drowning.
He saves Dedalus because despite his isolating tempests he's compassionate. He's able to feel his way past his own story into the stories of other people. Throughout the novel he charitably imagines what even his enemies might be feeling, and this merciful objectivity, as Iris Murdoch calls it, tends to make him forgiving.
Leopold Bloom is forgiving not merely in regard to other people. He is able to extend forgiveness to himself.
These abilities represent an earned hopefulness, not a glossy happyface.
A good part of the chapter involves Bloom eating at a hotel and listening to various Dubliners sing at the hotel's piano. He notes again and again our powerful reactions to music, and, with his restless curiosity, tries to account for music's power.
Here he responds to the very first notes from Simon Dedalus, Stephen's father, and, like Stephen, an excellent tenor:
Through the hush of air a voice sang to them, low, not rain, not leaves in murmur, like no voice of strings of reeds or what doyoucallthem dulcimers, touching their still ears with words, still hearts of their each his remembered lives. Good, good to hear: sorrow from them each seemed to from both depart when first they heard. When first they saw, lost Richie, Poldy, mercy of beauty...
He and his table mate, Richie Goulding, another sad, past-obsessed soul, hear this quintessentially human sound, the inimitable human voice, and even as the voice rouses them to "each his remembered lives," its merciful beauty relieves them of sorrow. It clears the confetti.
And that is why we love Leopold Bloom. Bloom is the primary singer in James Joyce's long libretto of merciful, euphoria-making beauty.