What plans does Pip believe Miss Havisham has for him? Why does he believe this?

In addition to his belief that Miss Havisham is to be his
benefactor, Pip also entertains the idea that Miss Havisham intends for him to marry
Estella.  In Chapters 11-13 of Stage One of Great Expectations, Pip uses his ambiguous
relationship with Miss Havisham--he wheels her around in her chair, he visits regularly
and plays with Estella--to entertain such a hope.  When Estella kisses him after he
defeats the pale young gentleman, Pip believes that Estella cares for
him.

In his self-deception, Pip interprets any time that he
is sent for or directed to meet Estella's carriage and escort her as the intentions of
Miss Havisham to have them marry.  Even in a later chapter when Pip takes Estella to
visit Miss Havisham at Satis House where Miss Havisham wickedly asks Pip, "How does she
use you?" and Pip remarks that he understad that Estella is set to wreak revenge upon me
for Miss Havisham, Pip, in his persistent delusion
declares,

I,
too was tormented even while the prize was reserved for me.  I saw in this the reason
for my being staved off so long, and the reason for my late guardian's declining to
commit himself to the formal knowledge of such a
scheme.