Because if you get sick you can pass it on to the elderly and the weak. You could be out in public and pass by someone with cystic fibrosis whose immune system is suppressed because they had to get a lung transplant or die and because you opted not to get the flu vaccine you got them sick and they die from a result of complications.

Kind of extreme, but entirely plausible. If you work/live with or around anyone who could be at risk then you definitely need to be vaccinated. If you don't I wouldn't say that it's mandatory... But you could get someone else seriously ill without even being aware of it.