I see it too. It is impossible to describe how popular he was in this period: he and his brothers were on tv constantly, plus making records and doing Vegas and international tours and promos and whatever else anyone wanted them to do. In the US in the early to mid 70s you literally could not go a week without hearing or seeing a reference to the Jackson 5 and especially Michael.

But what we didn't see at the time is that he and his brothers were effectively slaves, with their dad signing them up wherever he could make a dime out of it while the kids just worked and worked and worked, plus they had to be nice to everyone, could never let their guard down, could never just be kids, could never really personally enjoy any of the money everyone else was making off them at the time. They weren't even allowed any creative say in their earlier records: it was a shut up and sing situation.

So in this picture, Michael was exhausted and his father was still running his life, a relationship he always called physically and emotionally abusive in later years, as did his sister LaToya.

It doesn't excuse whatever he did later in life, but he looks a bit sad and tired probably because he was.