A good heart is the support for, and the most important part of, a person. Possessions can be lost then replaced, while the heart cannot be replaced. After the prince and the swallow died, there was no one else in the city that was as selfless as them. Once they were gone no one else in the city could replace their kindness. This idea is demonstrated in a short story by Eudora Welty, which also proves that genuine goodness is the base of life.
All these questions, including the troubles Keats was facing, caused him to go in and out of depression. Keats, bombarded with all these difficulties in his life, as well as his doubts about whether he was doing the right thing, was still able to become one of the major poets of the Romantic period.