Sarah and Arlo made their life choices before Spence's illness was even known, so I don't think that it affected the choices that they made. They each regretted, however, that they could not help to the extent they wished that they could, at the time it was all evolving. It did distract Sarah and she had trouble dealing with accepting and discussing it with her friends at school. Arlo's relationship issues with Spence appeared to be so deep but he was able to use the fruits of his labor to help in the long run. Arlo likely regrets not ever having been able to crack the outer shell into Spence's world, or solving the puzzle of lack of acceptance between them. Sarah perhaps regrets not having had a closer relationship with Arlo over the years. There are many relationships that have helped me progress thus far in this life - some smooth and happy, some rocky, but all with a forward propelling effect.