Maria's Contribution to the Animal Shelter

Read the following passage and answer the question that follows:

Maria is a dedicated volunteer at the local animal shelter. Every Saturday, she spends several hours helping to care for the animals. She cleans their living spaces, feeds them, and even takes some of them for walks. Maria enjoys her time at the shelter because she loves animals and believes they deserve a safe and loving environment. The joy she feels when a pet is adopted makes all her efforts worthwhile.

What is the most important aspect of Maria's contribution to the shelter?