When you provide leadership. In short, when you get outside your comfort zone. You have likely spent most of your life so far with persons pretty much like yourself and in a small set of environments. College and community activities, work, and volunteer opportunities offer you a wonderful way to experience diverse challenges that will promote personal development. Incorporate time in your schedule to develop yourself.
Study abroad is one way that students have found to almost guarantee a burst of personal growth.
Abroad you must navigate a new environment, rely on yourself, reflect on all that is new, and measure your own values. And don't forget your summers. That wonderful break between concentrated academic periods can be an opportunity to experience new things and confront new challenges. You should think long and hard about taking courses during the summer. Each summer spent in the classroom is a summer lost to new experience and the growth that comes from it.