We aren't catching the same virus infection every year. Viruses trigger an immune response. An immune response triggers symptoms. Because we maintain the same immune system each year (we don't shed the old one to generate a replacement) we experience the same symptoms each year. Every person's immune system is slightly different which is why people will have different symptom severity. Keeping the same immune system each year is why a lot of vaccines are one time actions or only once every however many years (also because some viruses/infections don't mutate year to year) and our body has the data ready to fight off a return of that virus/infection.

By the time you experience symptoms, the infection has already been in your body and you're been potentially transmitting it to others. Symptoms are the body reacting to the activities of the immune system attempting to fight the infection. It's why people with low or no immune system won't experience fevers, diarrhoea, headaches, lethargy in the same way others with a healthy system do. They'll go straight to life threatening status or death.