Theory:

     And the Giant’s heart melted as he looked out. “How selfish I have been!” he said; “now I know why the Spring would not come here. I will put that poor little boy on the top of the tree, and then I will knock down the wall, and my garden shall be the children’s playground for ever and ever.” He was really very sorry for what he had done.
Explanation:
 
The giant was pleased to look at that beautiful sight on the spring morning. He was surprised with his garden's ambience filled with a pleasant fragrance, sweet music of the linnets and delightful kids. Only after having seen this sight, he realised his mistake for being selfish all these days. "How selfish I have been!" he thought to himself. He then understood the reason as to why the spring had not entered his garden despite occupying the whole country.

Having regretted his mistake, he wanted to do something. Also, he desired to capture the spring in his garden.

He decided to help the little boy who was crying under the frozen tree. He thought of putting him onto the tree branch to play. In this way, he felt, spring would come back to that tree. Moreover, he was much determined to break the walls and to let children enjoy the joy of the garden forever.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Knock down To deliberately destroy a wall or building
Reference:
National Council of Educational Research and Training (2008). The Selfish Giant – Oscar Wilde (pp. 17-24). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.