Theory:

The story "A Tiger in the house" by Ruskin Bond, deals with various themes like love, connection, sacrifice and human-animal relationships. A tiger is supposed to be in the wild and it is naturally made to hunt and eat by the creator. The story deals with the issue of humans interrupting nature's flow. The tiger is calm and is conditioned by the narrator's family. But they could only maintain this characteristic to some extent because when the tiger grows up, its natural instincts also grows within it. Although it loves people and gets used to humans, he tends to go back to its roots, proving that no matter how much effort is exerted by the humans, nature has its own regulations and cannot be changed to one's own will. The ecosystem and the bond with the universe will be destroyed if one tries to do so. Each entity has a place within the ecosystem and one must stick to it.
 
The grandfather is also another example of how humans tend to escape reality by ignoring the truth. He addresses the tiger as Timothy even after the keeper informs him that Timothy had died of Pneumonia. Grandfather probably wanted to look at things from a positive side and chose to believe that the tiger had indeed loved him. He wanted to believe that the tiger licked his hand as a reciprocation of the love that he showed to the tiger. He believes that when one shows love, it will always be paid back. He sees his beloved Timothy in the form of the new tiger. He, therefore, addresses the new tiger as 'Timothy' and bids goodbye to him.