Some people who test positive for HIV were not aware of their risk. That’s why CDC recommends that providers in all health care settings make HIV testing a routine part of medical care for patients aged 13 to 64, unless the patient declines (opts out). This practice would get more people tested and help reduce the stigma around testing.
Even if you have been in a long-term relationship with one person, you should find out for sure whether you or your partner has HIV. If you are both HIV-negative and you both stay faithful (monogamous) and do not have other risks for HIV infection, then you probably won’t need another HIV test unless your situation changes.