why do we need to force everyone to be vaccinated

It's still your choice as long as you can live with consequences, so "force" may not be exactly the right word. No one is pointing a gun at you.

Unvaccinated people get sicker. They carry a high viral load longer and pass the virus on more often and more easily. This provides more opportunities for the virus to mutate, producing variants. There will always be people who can't get vaccinated and others who are more vulnerable to the virus for other reasons, so anyone who can get vaccinated should do so to help reduce the impact of the virus on those who are vulnerable, not to mention the economy.

Remember that all ages get the virus. See Figure 4 on this page: https://health-infobase.canada.ca/covid-19/epidemiological-summary-covid-19-cases.html#a5. Young people may have a lower risk but they are a bigger threat to others if they are unvaccinated..

Overall vaccinated people don't get as sick and don't die as often. They place less of a load on tattered healthcare systems. When they are sick their viral load decreases faster and there is evidence that what they do pass on isn't as infectious, so they are less likely to infect others.

Most of these benefits and more are listed here: https://www.cdc.gov/coronavirus/2019-ncov/vaccines/vaccine-benefits.html. I'd be happy to provide more links if you have more questions after reading that one.