If what you're hearing over and over is the Piano Concerto No 2 and Prelude in C sharp minor, then no wonder you've got that opinion. But the truth is that Rachmaninov was a talented composer of a quite varied body of musical output. A lot of folks find death as a common thread in his music - and it certainly is there, in the Paganini Rhapsody, in the Isle of the Dead, maybe in a couple preludes. A lot of folks find needless virtuosity in his music - well, he earned his pay as a virtuoso concert pianist and, in the grand old tradition of Chopin and Liszt (whose concertos have far less substance, in my opinion), he wrote music to play in concert.
That said, I think there is a general tendency to fixate on Rachmaninov's very obvious emotionality and the sorrows of his personal life and use them to create a narrative that's dismissive of him as a serious, intelligent composer. You don't have to buy that narrative. I don't. I think his 24 preludes for solo piano are varied and interesting, an intelligent cycle of piano works that stubbornly refuses to indulge in what you call self-pity. Branch out beyond the famous stuff - or even look at the famous stuff from a new perspective, like the stark minimalist intensity of the Isle of the Dead or contrapuntal talents on display in the Symphony No 2.
Rachmaninov is not necessarily one of my very favorite composers (those are Beethoven, Dvorak, Shostakovich, maybe Chopin) but he is one I admire and greatly enjoy. He is unafraid to wear his heart on his sleeve, and that heart happened to be a big one. But I can't say I detect "two or three self-pitying ideas" wending their ways through his work as a common thread or a recyclement. Maybe the only common thread I see is that a lot of his finales (Piano Concertos 2 and 3, Symphony 2) tend to end with the subsidiary theme sweeping through the string section in slow-motion, followed by one last cascade of quick little notes.
