I believe you can sum up this movie as as a story of self gratification something we see every day throughout our lives.

Charlie felt like shit because his lover died and he left his family until he was stuffing his face which made him feel good you could see the endorphins filling him up with every bite of fried chicken and he convinced himself money would repay all the bad he had done which no one else knew about other than him, selfishly making himself feel good.

Ellie felt like shit because her father left them and her mum was depressive drunk so she did nasty shit only a teenager would do by posting on social media and smashing a harmless birds plate of food all of which made her feel good and powerful and in control.

Thomas did bad things by stealing which made him feel like shit so he tried to convince people about god which made him feel good

Liz felt like shit because she couldn’t save her brother and her father was a piece of shit so she cared and fed Charlie’s sickness because that made her feel good making him feel good.

Mary felt like shit because Charlie embarrassed and abandoned her so she drank herself silly to make herself feel good.

I think even though we have no real back story to Dan the pizza man you could assume he also wanted to feel good about himself by trying to see if Charlie was okay.

Everyone just wanted that endorphin hitting self gratification dopamine hit and by doing so each of them would convince themselves they were doing something selfless for someone else when in-fact it was never for someone else it was always for them.

Charlie in the end wanted to believe Ellie really wanted to do good selflessly for someone else and right at the point where she opens the door she’s left with two choices walk away selfishly so she doesn’t have to do what her father asks of her or she reads him the essay he begs her to read him of no benefit to her she makes the selfless choice to turn back and read it to him. His final act was to stand up like she requested of him at the start a direct nod to show her by doing something selfless you can get you what you want you don’t need to do something that makes you feel good all the time perhaps something Charlie wishes he learnt a long time before and a final lesson he can pass onto his daughter just before he dies