Character Writing 101: 'Want vs Need'.
The 'want' is what the character desires and their pursuit of the want drives the A plot of the story. The 'need' is what the character requires to fulfill the arc driving the B plot. The conflict between want and need comes to a head in the climax, with the character embracing need over want and achieving catharsis for the audience.
In Up, Carl wants to be left alone with his possessions to grieve his wife. Carl needs to let go and move on. This conflict comes to a head in the final act when Carl chooses to literally let go of his house (the physical embodiment of his grief for Ellie) in order to save Russel, Dug, and Kevin. Even if Carl somehow found a way to save them without sacrificing the house, it wouldn't have had as much of an impact.
In Beauty and the Beast, Beast wants to make Belle fall in love with him and break the curse. Beast needs to learn to show kindness to others and put others before himself. This conflict comes to a head when Beast chooses to let Belle and see her father even though it might mean she'll never return and the curse will never be broken. This decision leads to Belle realizing she truly cares for Beast, returning to help him, and professing her love for him and breaking the curse. Even if Beast found some way to help Belle's father it wouldn't have had as much of an impact if he hadn't done the selfless thing and let her leave.
Jack wants to get back to the past and undo the future that is Aku. Jack needs to forgive himself for failing to save the past and realize how much he has to fight for in the future - and indeed this dynamic is examined throughout the series and especially in the fifth season. This conflict *should* have come to a head in the climax of the finale, with Jack ultimately choosing to stay and fight for the future instead of going back to the past. This didn't happen, and so the ending falls flat for a lot of people because there's no catharsis for Jack's arc