Sometimes people find that the ambitions they had in their 20s fade in their 30s and life can feel empty in their 40s. Others can find that over time those things that provided stimulation also fade. Even people who raised families can find that their grown children move away and they also become detached.
One can revisit old pleasures like a favorite book or movie. However, exploring new interests like taking a leisure learning class, joining a book club, or dabbling with marksmanship, bowling, or amateur radio might uncover new interests. Sometimes finding out what groups meet at the local library can open options.
Another line of inquiry can be volunteer activities. Reading to the elderly, cleaning up litter, or even being a mentor might make some connections through which a vitality and zest for life can be restored.
Suicidally Curious is right about the idea of life as a construct or product. Too often people are brought to dissatisfaction with their life because it falls short of what has been presented in TV and movies. Satisfaction does not generally lie in the direction of trying to live someone else's idea of life. If you explore and experimenter, you may find what is right for you.