Was it a single event or a long term volunteering commitment? Sort of hard to tell based on your description but it might be something like this? I absolutely loved being a teaching assistant in college and it was a very positive and meaningful experience for me overall. However, I wrote several essays on a challenging situation that involved working with others. That experience, however, does not change the fact that I loved my role as TA throughout my three years. Having a meaningful situation that was extremely positive and influential does not mean that there weren't moments or aspects that were challenging. In fact, it's likely that those lessons learned contribute to how positive an experience that was because you gained insight and skills.