The idea of enjoying life more but dying earlier sounds reasonable in theory but in practice, the things that make you live into your 90s are the same things that make you able to be healthy enough to continue to have an enjoyable life in your 40s-70s, so unless you're planning on dying of a heart attack in your 30s there isn't necessary as much of a trade off as you might think.

In your fantasy world people must be really happy every day, exercising every day and eating very healthy.

Do you mean simply because people outlive their friends? If so, do you think simply having friends that are younger, and possibly still working and having colleagues till a late age, would fix this? Doc Severinsen seems happy in his 90s, outwardly at least. Who knows if he's content behind closed doors. I know a lady from local swimming pool who swims daily who's 91. She's slightly falling apart to be honest ;) But happy and has friends. EDITED or perhaps you mean the many in poor health in care homes who are bored?