Lots of people get pleasure from shopping, and in itself it is not a problem.
Retail therapy becomes a problem in one of two ways. The first problem is if the person gains pleasure and satisfaction from their purchases and so is indulging in comfort shopping. This tends to lead to maxed out credit cards and a long term sense of dissatisfaction because owning things is not a substitute for long term goals. The second problem is if the person doesn't get pleasure and satisfaction from the purchases, arriving home with the shopping only to discard it untouched. Compulsive shopping is often a very impulsive behaviour.