The three stages I see as a reader include these:
1. Love: In the beginning I saw a brother motivated to help his learn to be more than he was. This love may have been selfish because all boys want someone to play with.
2. Cruel Pride: It seems that every time Doodle achieved much more than would have been expected from him, it was not enough. When the experts though he would never move and Doodle crawled, brother wanted him to walk. This stage continued all the way until he abandoned Doodle in the rain on that last day. We see this concept exemplified in the quote:
There is inside me (and with sadness I have seen it in others) a knot of cruelty borne by the stream of love.
3. Guilt: I think the purpose of the story is to show the guilt the author felt for abandoning his brother, one who was less capable of handling himself than the average person.
I see these stages connect the the theme of the conflict between love and pride. Doodle's brother wanted to love him all the time, but it was a burden to bear all of the effort that it took to ensure that Doodle acquired skills necessary for life. The one day that brother abandoned Doodle after getting fed up with helping so much was the day that he needed the most help.