Dill comes from a broken home. His mother and father are divorced (or at least separated); the mother works and has little time for Dill. As soon as school is out, they send Dill off to Maycomb to spend the summer with his Aunt Rachel. Dill has a strong need for belonging, and Jem and Scout soon become his closest friends. Dill and Scout engage in a summer full of puppy love, and they become engaged (sort of):
He had asked me earlier in the summer to marry him, then he promptly forgot about it... said I was the only girl he'd ever love, then he neglected me. I beat him up twice, but it did no good. He only grew closer to Jem. (Chapter 5)
Boys will be boys, and despite his attraction to Scout, Dill still gravitates toward the older Jem. It is not really surprising that the two boys hit it off, leaving the lone girl as a "third party." But it is ironic that Dill would treat his "permanent fiance" in such a manner since his "neglect" of Scout is reminiscent of his own unhappy family life, in which his parents ignore him. But after all, Dill and Scout are children, and their attention spans are short. In Scout's case, Dill's absence only makes the heart grow fonder.
Summer was Dill... eyes alive with complicated plans... the swiftness with which Dill would reach up and kiss me when Jem was not looking... the longings we sometimes felt each other feel. With him, life was routine; without him, life was miserable. I stayed miserable for two days. (Chapter 12)
Dill's neglect provides Scout with a reason to become closer to her neighbor, Miss Maudie. Spending the early summer evenings on Maudie's front porch, their friendship becomes "cemented," providing Scout with a future female role model.