You rejoice, Holly, that you've done something right. Time-out, as a disciplinary technique, is intended as an alternative to spanking or hitting a misbehaving child. Although many children find it very unpleasant and undoubtedly interpret it as punishment, that is not its real purpose. Its purpose is to help children learn—learn to recognize unacceptable behavior and learn that such behavior will result in isolation from loved people and interesting activities.
I wouldn't worry that your little girl seems to enjoy time-out. You didn't mention her age, but I'm guessing that she is two or three. If two, when you let her out of time-out, repeat to her why she was put there in the first place. If three, try to have her verbalize what it was that led to the isolation. If she can't do it, help her.
And you should feel good that she puts herself in time-out when she misbehaves. This shows that she understands what the procedure is used for and is trying to self-correct her misbehavior. That is the goal of good discipline—self-correction and self-guidance. It also shows that she is modeling her behavior after yours—and that you are modeling appropriate behavior for her.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.