Looking through some old scores, I come across the following note written by one Edouard Lindenberg, and copyrighted 1951:

Schumann said of Mendelssohn that his first name, Felix (happy) suited him admirably.  Mendelssohn was, in fact, of a carefree disposition, full of gaiety and optimism, and he was spared material cares.  Sorrow almost always passed him by – he never experienced any really severe shocks of any kind. Is it on this account that his music never attains the highest summits?  Or was it perhaps that he was too universally gifted – for he spoke several languages, read Greek fluently and had translated Terence; he was, moreover, one of Hegel’s best pupils and his talent as a draughtsman and painter in watercolours was very superior to that of an ordinary amateur.”

What an amazing person Lindenberg must have been!  He was clearly deaf, because even a short acquaintance with Mendelssohn’s music would tell you that the composer had experienced profound sorrows and emotions, and had expressed these in his music.  Listen to his last quartet, written after the death of his sister, Fanny, for example, or the two violin concertos.  Or listen to the opening orchestral number of the oratorio Elijah, which, again and again and again, seems about to resolve but has its resolution postponed, thereby expressing  human anguish better than any other composer before or since.
But my amazement at this man Lindenberg is even stronger.  He must also have been blind as well as deaf, since his concert note (concerning the Hebrides overture) then quotes from a letter Mendelssohn wrote from Paris on 21 January 1832.

I cannot have the Hebrides played here because I don’t consider the work finished yet.  The central section ff in D major is very stupid, and the whole development smells more of counterpoint than of seagulls and fish – whereas it should be the other way round.  And I am too fond of it to be played as is stands.”

But just two weeks later Mendelssohn wrote the following letter, immediately after hearing of the unexpected death from tuberculosis of his violin teacher and close friend, Eduard Rietz (1802-1832).  Surely, unless he was blind, Lindenberg must have also seen this, the very next letter in the published edition of Mendelssohn’s letters:

You will, I am sure, excuse my writing you only a few words to-day:  it is but yesterday that I heard of my irreparable loss.  Many hopes, and a pleasant bright period of my life have departed with him, and I never again can feel so happy.  I must now set about forming new plans, and building fresh castles in the air; the former ones are irrevocably gone, for he was interwoven [page-break] with them all.  As I shall never be able to think of my boyish days, nor of the ensuing ones, without connecting him with them, so I had hoped, till now, that it might be the same with those to come. I must endeavour to inure myself to this, but the  fact that I can recall no one thing without being reminded of him, that I shall never hear music, or write it, without thinking of  him, doubles the sorrow of such a separation.  The former days are now indeed departed, but it is not these alone that I lose, but also the man I so sincerely loved.  Had I never had any, or had I lost all cause for loving him, I must without a cause have loved him all the same. He loved me too, and the knowledge that there was such a man in the world – one on whom I could rely, who lived to love me, and whose wishes and aims were identical with my own – this is all over: it is the hardest blow that has yet befallen me, and never shall I forget it.
This was the celebration of my birthday.  When I was listening to Baillot on Tuesday, and said to Hiller that I only knew one violinist who could play the music I loved for me, L______ was standing beside me, and knew what had happened, but did not give me the letter. He was not aware indeed that yesterday was my birthday, but he broke it to me by degrees yesterday morning, and then I recalled previous anniversaries, and took a review of the past, as every one should on his birthday; I remembered how invariably on this day he arrived with some special gift which he had long [page-break] thought of, and which was always as pleasing, and agreeable, and welcome as himself.  My day was very sad; I could neither do anything, not think of anything, but the one subject.
To-day I have compelled myself to work, and succeeded.  My overture in A minor is finished.  I think of writing some pieces here, which will be well remunerated.
I beg you will tell me every particular about him, and every detail, no matter how trifling; it will be a comfort to me to hear of him once more.  The octet parts, so neatly copied by him, are lying before me at this moment, and remind me of him.   I hope shortly to recover my usual spirits, and to be able to write to you cheerfully and more at length.  A new chapter in my life has begun, but as yet there is no title.
— Your Felix. ”

[Mendelssohn-Bartholdy 1864/1870, pp. 327-329, letter from Paris (to Fanny?), dated 1832-02-04]

To imagine that a privileged person does not suffer normal human sorrows in the same way that the rest of us do is a peculiar form of irrationality, contrary to all human experience.  To further imagine that such a person is not capable of profound artistic expression despite the evidence of own’s own senses is just perverse.  But Mendelssohn seems to have attracted the perverse among his critics, from the explicit anti-semitism of Richard Wagner to the anti-Victorianism (and possible anti-semitism) of George Bernard Shaw.
References:
Edouard Lindenberg [1951]: F. Mendelssohn  Fingal’s Cave (Hebrides Overture) Op. 26. Paris, France:  Heugel & Companie.
Felix Mendelssohn-Bartholdy [1864/1870]: Letters from Italy and Switzerland. Translated by Grace, Lady Wallace.  Fifth Edition. London, UK:   Longman, Green, Reader, and Dyer, 1870.  Includes preface to the First Edition, dated 1864-04-22.