Why can't Superman be complex and happy? Have a heart of gold and unrelenting morals but still have his own struggles? Being a grim, brooding asshole doesn't really make anyone more "realistic", but Superman isn't supposed to be realistic anyway. He's literally supposed to be someone the world can look up to, who does the right thing even if its hard or unpopular. His struggles are about living his life outside the suit, things like how he balances having a life along with his self imposed responsibilities to the world as Superman. He has the powers of a god but he is a man behind the smile, and thats what makes him compelling.
The comic story "What's So Funny About Truth, Justice & the American Way?", and its animated adaptation Superman vs. The Elite, is basically the perfect Superman story to show who he is and why he acts the way he does. Superman seems to be a "Smiling Joe" (whatever the fuck that condescending nickname is supposed to mean) because thats what the world needs, and its not easy, even for him.
Unfortunately that kind of complexity is too hard for many writers to understand, so we've gotten this whole movement of assshole and/or evil Supermen stories because of a lack of understanding of what makes Superman special, and the belief that being a violent asshole is more relatable, missing the entire point of Superman.