build a fishing boat with his new father. Here is the quote from the book:

But summer came and Dill was not there. I received a letter and a snapshot from him. The letter said he had a new father whose picture was enclosed, and he would have to stay in Meridian because they planned to build a fishing boat. His father was a lawyer like Atticus, only much younger. Dill’s new father had a pleasant face, which made me glad Dill had captured him, but I was crushed. Dill concluded by saying he would love me forever and not to worry, he would come get me and marry me as soon as he got enough money together, so please write.

As we can see, in chapter 12, Dill and Scout have a special friendship. Dill really does like Scout a lot. Later on, Scout admits that she had a hard time knowing that Dill would not be with them for the summer. She writes:

The fact that I had a permanent fiancé was little compensation for his absence:

Also what made things harder for Scout was that Atticus had to travel for the next two weeks back and forth to the state legislature, which was called into session. 

Dill writes Scout a letter at the beginning of the chapter telling her that he has a new father, and they are going to build a boat together.  He asks Scout to write him, and Scout is disappointed Dill won’t be visiting her because Jem is being cantankerous and telling Scout she needs to act more like a girl. Perhaps Dill has found the father he has been looking for. 

Dill misses out on some excitement when he is unable to visit Maycomb that summer.  First of all, Calpurnia takes Scout and Jem to her church while Atticus is out of town on state business.  Scout and Jem also learn about Bob Ewell’s accusations that Tom Robinson raped Mayella.  The congregation collects $10.00 for the Robinson family, and Scout and Jem chip in a few dimes to the cause.  Scout learns a lot about Calpurnia during this chapter like why she talks “different” around other black people.  We see in this lesson Scout learns that Calpurnia has to navigate two different cultures, and her speech changes due to which culture she is experiencing.  At the end of the chapter, Aunt Alexandra arrives as well, and Scout is not happy!

    The summer begins badly for Scout in Chapter 12 of Harper Lee's novel, To Kill a Mockingbird. Jem is "growing up" and hasn't much time for his sister, so Scout spends time in the kitchen with Calpurnia until it is time for Dill to arrive.

    But summer came and Dill was not there.

When Scout receives a letter from Dill, she discovers that he has a new father. He is a lawyer, like Atticus, and they will have to stay in Meridian, Mississippi to build a boat. Scout is happy for Dill, whose parents rarely spend much time with him, but she is crushed that her fiance will not be there with her.
    Of course, Dill eventually shows up, surprising Jem and Scout with his appearance but not with his new array of stories.

At the beginning of chapter twelve, Scout laments about the growing divide between her and Jem before mentioning that Dill did not arrive during the summer. She had received a letter from Dill explaining why he would not be around during the summer. According to the letter, Dill had gotten a new stepfather and could not spend the summer in Maycomb because he had to help his stepfather build a fishing boat. Dill then tells Scout that his new father is similar to Atticus, only much younger, and has a pleasant face. Given Dill's massive imagination and capacity for telling tales, it is difficult to tell exactly why he wasn't able to spend the summer in Maycomb. One could surmise that Dill is more than likely living with another relative since he has spent previous summers with Rachel Haverford, who is a notorious drinker. It is also possible that Dill's mother felt that it would be best for him to get to know his new stepfather. It is doubtful that Dill spends the summer building a fishing boat with his stepfather because in chapter 14 he runs away from home and explains to Scout that his parents ignored him.

To Scout, Dill meant summertime. She looked forward to him being there to play with, and to be her fiancee'. On page 117, Scout narrates to the reading audience:

I received a letter and a snapshot from him. The letter said he had a new father whose picture was enclosed, and he would have to stay in Meridian because they planned to build a fishing boat. His father was a lawyer like Atticus, only much younger. Dill’s new father had a pleasant face, which made me glad Dill had captured him, but I was crushed. Dill concluded by saying he would love me forever and not to worry, he would come get me and marry me as soon as he got enough money together, so please write.

Remember, Scout is a narrator that is very young and Dill is a kid that lies. These contents of his letter make Scout feel like there are reasons that he isn't coming. From what we as readers know about him, we can see that he moves from relative to relative, so we can't always believe what we hear.

Dill does not come for the summer because his mother remarried.

When summer comes at the beginning of Part Two, Scout contemplates how her “permanent fiancé” Dill does not come.

I received a letter and a snapshot from him. The letter said he had a new father whose picture was enclosed, and he would have to stay in Meridian because they planned to build a fishing boat. (ch 12)

Dill does send a picture, and his new father has “pleasant face.”  Scout is upset because Dill said he would marry her, and then he does not come.  Scout enjoys the summers with Dill, and summer will not be summer without him.  Asking Scout to marry him is just his child's way of trying to get himself a family.

Dill does show up, and Scout finds him hiding under her bed.  He has a story about how his new father chained him in the basement and he escaped and took the train, but in reality he just felt that no one needed him.

"Well, they stayed gone all the time, and when they were home, even, they'd get off in a room by themselves." (ch 14)

Dill comments that his new father did not build the boat like he promised, and his parents just sat in a room alone reading, and didn’t want him with them.  Lonely, he had headed to Maycomb where some people did care.

Dill’s escape demonstrates how the Finches are more of family for him than his own.  His mother has no time for him, and his new father makes promises he can’t keep.  Dill lives for summer, the only time when he feels a sense of belonging.