If your body doesn't hold the immunity it's easy to get them again. my father had them 3 times before he was 15.
I however had 6 visible pox when I was 3, none on the scalp, none in my mouth. However my immunity level in 2004 was 11.6 & now is 11.9 (no idea how it went up I've not been around anyone with chicken pox)
I cannot hold Immunity from the MMR. I had it on my 15th birthday, & my 30th. I was surprised then I got pregnant a month after my 32nd that I had NO immunity, not just low.. None, like I never had the needle 2 years ago.
Everyone's bod is different.