Well, It's not grey for me.
Choose to help Eve and Jack will go out to the store. On his way back, he has the option to give Cassie a ride or not.
This option is greyed out because either you didn't help Eve, or you didn't give Cassie the ride she needed.
I hope that helps.
No worries.
There are a lot of options, some are significant, and some are not. Some will change things on a large scale, some will only impact the current dialogue. It's easy to get lost. I wanted it that way thinking it would push the player to play and replay the different paths in order to find new elements of the story he could have missed.
However, I also do understand that it might be a source of frustration. Especially when you feel like hitting the same wall multiple times.