Concise answer
Acne can recur because the underlying tendency remains, treatment was stopped early, triggers continue or the original condition was not acne alone.
Oil, follicular blockage, inflammation, hormones, products and friction can contribute in different combinations.
A maintenance plan may be needed after active acne improves.
When evaluation may help
Review is helpful for painful, scarring, persistent or sudden adult-onset acne and when hormonal symptoms are present.