Unfortunately, the whooping cough (Pertussis) vaccine is among the least effective, at only 80-90%, and protection begins to wane at around the 10-year mark: https://www.cdc.gov/pertussis/about/faqs.html
The previous version of the pertussis vaccine, previously given as the DTP (diphtheria, tetanus, and pertussis) was more effective, but the side effects weren't great, and people got whooping cough. You'll note that now, your doctor will recommend a DTaP booster - the lowercase "a" is for "acellular," which, while less effective, prevents transmission of the disease from the vaccine.
While no one in the history of time has ever thought it was fun to have a horrible cough for several months, most adults will weather whooping cough without serious complications or lasting damage. The primary reason to get boosters is to protect others - if you caught whooping cough, you might be fine, but transmitting it to a baby too young to be fully vaccinated, a person whose immune system is severely compromised (like someone on chemotherapy), or the elderly, who often have waning immune systems, could die.
A lot of children start participating in sports and camps that require a physical prior to participation. This is a great time to talk to your doctor about boosters. As a bonus, since the whooping cough vaccine is only given in the combination shot, your child will get a tetanus booster as well.