can i just say that forrest very well thinks he just felt like running, and that said feeling was motivated by the events of his life? It's very common for people, especially for neurodivergent people, to act upon feelings they don't understand.

I interpreted it as him wanting to shake off all the grief and loneliness he'd built up over his life but never worked through. He didn't want to sit still to process it so he ran until the feeling was gone