There are many reasons to join.
To start with, your supervisor today may not be your supervisor tomorrow. Without a union contract, you have no guarantee that your wages and working conditions will not be undercut by a new manager or supervisor.
Unions can provide dignity in the workplace by ensuring that the employee-employer relationship is not controlled by just one party. The best strength you and all Toyota Team Members can have is the strength you lend each other. If your supervisor genuinely respects you now, they will respect your right to choose a union. This choice will not damage a positive relationship but will in fact strengthen it.