I know it is not the politically correct thing to say on this board, but I don't prefer Mahler's music over Brahms and Beethoven. I enjoy his music, yes, when I am in the mood for his music. But I also like to listen to chamber music, and I like solo piano music, and concerti with piano, cello or violin, and I feel a strong affinity to Russian music, and some opera.
Mahler is just one composer, and unfortunately, a composer with a very limited purview.
However, having said all that, my answer to your question is -- I like Mahler because of his talent for making the "over the top" grandiose statement. He is so extreme, so invested in what he felt compelled to say musically, that some of his symphonies are just wondrous to experience. I don't want to hear them every day, or even every week. But when I'm in the mood for that sort of thing, no other composer will do.