While eNotes educators cannot write your essay for you, questions like yours are always welcome! I will endeavor to point out a few things which will greatly aid you in writing your essay.
To start, Helen's experiences with kind people in Boston have greatly influenced her thoughts about the city. Because of the people, Helen will forever equate Boston with kindness, welcome, and compassion. The main example of someone who embodies the spirit of such welcoming hospitality would be Mr. William Endicott. Helen tells us that she was thinking of Mr. Endicott when she called Boston The City Of Kind Hearts. Mr. Endicott opens up his house to Helen and talks to her as if they are great friends who have always enjoyed each other's company.
Another example you might consider using would be that of Mr. Anagnos (director of the Perkins Institution For The Blind in Boston). When Helen's father writes to request a teacher for Helen, we are told that Mr. Anagnos answers with a 'kind letter' filled with ' the comforting assurance that a teacher had been found.'
In May 1888, Helen gets a chance to visit the Perkins Institution For The Blind. Her is joy is complete when the children greet her with eagerness and enthusiasm. She feels so thoroughly at home in Boston that she begins to regard Boston 'as the beginning and the end of creation.'
Another instance of kindness is experienced in the presence of the child actress, Elsie Leslie, who stars in the play, 'The Prince and The Pauper,' in Boston. Despite fatigue, Elsie receives Helen with a generous warmth and a kind welcome after the play. Helen tells us that it 'would have been hard to find a lovelier or more lovable child than Elsie...'
Helen tells us of her sadness at the death of Mr. John Spaulding, a great supporter of Helen's studies.
Only those who knew and loved him best can understand what his friendship meant to me. He, who made every one happy in a beautiful, unobtrusive way, was most kind and tender to Miss Sullivan and me. So long as we felt his loving presence and knew that he took a watchful interest in our work, fraught with so many difficulties, we could not be discouraged. His going away left a vacancy in our lives that has never been filled.
Above, I have listed some examples of people who have contributed to Helen's great experiences in Boston. Because of these people, Helen thinks it is appropriate for Boston to be known as The City Of Kind Hearts. For your essay, you will probably find other examples you might like to use. I hope the above has helped!