The antibodies you develop to fight the virus fade over time. I just had it fairly recently (young 30s, vaccinated with the attenuated chickenpox virus, never had chickenpox, so this was likely the vaccine strain¹). Did a lot of reading and research during and after. The antibodies seem to offer good protection for 5-10 years following either vaccination or infection according to the literature I was reading.

¹ The vaccine strain tends to be much more mild than the wild strain, and indeed it was quite unpleasant, but not extremely painful for me. The wild strain is considerably more painful and linked to a greater incidence rate of complications. Please do not skip chickenpox vaccinations for your kids, the minor risk of latent infection from attenuated vaccine is far less harmful than the consequences of not vaccinating. Most important of all, if you have a cluster of blisters or rash on one side of your body that keep popping up, make sure to see a doctor and get on antivirals within the first 72 hours for best results.