In the mid-1840's, Mendelssohn was filled with despair. He, like everybody else, saw two secular churches besetting his era: the church of right-wing nationalism, and the church of left-wing social justice. Did watching these mirror images of each other take Europe by storm contribute to killing him? His beloved sister died six months earlier, he was utterly overworked, evidence after 150 years suddenly appeared that the scrupulously moral Mendelssohn had an affair with the famous soprano, Jenny Lind, and threatened suicide if she didn't agree to his plan for them to leave their families. To picture how shocking this revelation is, you have to realize that this was the BFF of Queen Victoria and Prince Albert.
Mendelssohn was an anachronism: perhaps too smart, and too privileged, for his own good. He was free from the concerns which beset most of his contemporaries, and because he was free of them, he unjustly saw further than they did.
He grew up in an eminent and wealthy financial family of assimilated Jews (who Baptized their son at birth), and while so many of the overprivileged like them put their privileges to no use, Mendelssohn used it to become a universal genius, brilliant in virtually every area in both the arts and the sciences. He could nearly as easily have been a brilliant novelist or painter or linguist or mathematician, and he seemed to settle upon music on a whim.
Many distrust Mendelssohn as a musical lightweight because his music is so much fun, but in fact, his music is as serious as seriousness can become - seriousness does not preclude fun, in many ways it can increase fun. Mendelssohn's seriousness was grounded in the fact that he was not interested in the lightweight diversions of his era, but only in listening to and studying the best of the best music, books, and art, regardless of era or nationality. Berlioz, when he first met Mendelssohn, said that Mendelssohn was great company but 'too fond of the dead.' There's something a little stultifying about Mendelssohn's kind of puritanism - sometimes we all need to indulge in a fondness for trash and filth. But when we see what happens when the world takes pride in its trash, we have to entertain that perhaps this snob had a point...
No matter what caused the strokes that killed Mendelssohn, despair was clearly part of the equation. Had he lived another year, he'd have seen the 1848 revolutions: Would the revolutions or their failure have depressed him more? The music from his final years has a Beethoven-like fury that is almost missing from earlier endeavors. Greatest, and most furious, of his late works is Elijah - a return to the religion of his grandfather to tell the story of the most furious of all the Old Testament prophets, and a barely disguised call to his audience to return from frivolity. Seventy years later, that striving for something more than bourgeois virtue which Mendelssohn so devotedly represented, would lead to a generation-long series of World Wars that completely killed the culture whose destruction he perhaps foresaw. Mendelssohn realized that such people gravitated to such utopias because they acted with insufficient seriousness, and because they were insufficiently engaged with learning, they gravitated to substitutes for religion with radical utopias. Before we even get to dream of a better world, we have to act sufficiently well that we deserve it. Why get rid of religion in the first place if you still act religious?