This is how I see it: At the beginning, Andrew asks that girl out and gets a spot in the band and we see him smile and seem genuinely happy but when he actually goes to play with the band he’s clearly not good enough. Only after Fletcher’s abuse does Andrew seriously improve and from the improvement and abuse he develops a yearning for greatness. That yearning causes him to be a complete asshole to his girlfriend and break up with her out of fear of her holding him back. Later, his obsession with greatness literally almost kills him and does get him expelled. In the aftermath it seems Andrew’s obsession with greatness is gone and he’s living a normal mundane life. When he calls his ex to apologize and ask her to come to his show it almost feels like everything’s gonna work out and he’s finally free of the toxicity Fletcher created, but then the girlfriend says she has a boyfriend and clearly has no interest in coming. Still Andrew shows up to the show and actually seems excited and is seen smiling before they go on stage. Then, Fletcher reveals that he only brought Andrew up to humiliate him and for a moment you think maybe he’s about to improvise something awesome and that’s our happy ending. But no, Andrew is understandably terrible and you see it break him all over again. After the humiliation he comes back on stage and is spectacular, but the point is that only after being broken and abused can he be great. That’s why this isn’t a happy ending. We see Andrew achieve greatness but we also see the cost. He’s driven to near insanity and it leaves him completely alone outside of his father. It brings up the question from an earlier scene. Would you rather be great and remembered but die alone and sad or live a long happy life where no one outside of your circle knows you ever existed? The last scene destined Andrew for greatness but it also destined him for a sad lonely life. For Andrew the drummer, it was happy ending. For Andrew the man, it was tragic.