How is the theme of friendship shown in Stuart Little?

Stuart Little by E.B. White is a charming, but unusual, tale of a little mouse adopted into a human family. Although Stuart wants nothing more than to fit in with his family, he is sometimes taken advantage of for his size when family members want him to do various tasks for them, like going into a drain to fetch a lost ring. His big brother, George, holds resentment for Stuart, making his attempts to fit in with the family even more difficult. And the cat, Snowbell, a mouse’s natural predator, is not interested in any type of friendship despite Stuart’s best efforts, instead trying to plot ways to get rid of Stuart.
It is when Stuart decides to strike out on his own in search of companionship that he ultimately finds the friendship he craves in the unlikeliest of places. First, he befriends a surgeon-dentist named Paul Carey, who gives him a position on his model schooner, allowing Stuart to experience the exciting thrill of a model boat race that is filled with adventure. He chooses not to share the adventure with his adopted family, the Littles, preferring to keep the treasured experience and friendship to himself.
Later, Stuart saves a foundling bird named Margalo from Snowbell’s advances, securing a deep and lasting friendship that brightens Stuart’s lonely days of attempting to fit into a family that is an entirely different species from him and cannot understand his needs. Margalo also saves Stuart from impending death, returning the favor, and the two animals watch out for each other in a world filled with predators, further securing their friendship.
Stuart is crushed when Margalo disappears, and he gratefully uses the gift of a little car bestowed upon him by his other friend, Dr. Carey, to search the dangerous city for her. In his search for his best friend, Stuart also befriends a girl named Harriet who is a potential mate for him. But his friendship with Margalo is too important for him to consider settling down, and he willingly leaves his potential mate behind as he continues to search for her, choosing loyalty for a proven friendship over a possible match. His diligence pays off, and he eventually achieves his goals and saves Margalo once again, bringing her home.
When Stuart strikes out on his own to find Margalo and leaves his adopted family behind, Stuart’s brother George realizes how much he really cares for Stuart, and even the cat, Snowbell, realizes he really cares for the mouse.
In the end, not only is Stuart surrounded by his older friends, but his brother George and Snowbell, showing that a kind and heroic heart can overcome any obstacle, reaching people in all walks of life. All of Stuart’s friends were of a different species, but that did not matter to the brave-hearted little mouse. He remained true to those he considered to be a friend, going above and beyond the expectations of friendship in his loyalty. Because of his loyalty, he was able to show George and Snowbell the meaning of true friendship, and the entire experience brought his good friends and adopted family together in a special way.