So what happens when you have no interests, no hobbies, no cause? And for those that do, but lack the time or financial resources to persue them? What then?
If you lack the time, then your problem is one of not having proper work/life balance. If you lack the financial resources to come up with enough money to go bowling once a week then your problems are deeper then not having friends.
if you lack financial resources, that's the problem to tackle. Making friends and having leisure activities will be secondary until your financials become stable and good.
Once you have the financial resources, _then_ you tackle the problem of no interest/hobbies/causes issue - which, i say is relatively easy to tackle as long as you have money; try different things until something sticks!
Slight miscommunication here. I'm talking about someone that genuinely has absolutely no interests whatsoever. Even after trying skiing, bowling, hiking, just about every time sport, reading, music in all forms, performance arts, painting, board games, table top games, video games, travel, etc. And everything is dull, boring, uninteresting, and unejoyable.
In that case, this person might actually have a medical condition of some sort. Humans tend to be curious and that curiosity is manifested as an interest in something.
Either they have not tried everything, or they have imbalances or an illness that makes them feel this way. If this is true, there should be no shame in visiting a doctor or therapist of some sort. Otherwise, depression would set in sooner or later and get worse.