Let me start by saying Branch is my soul-troll. He's described as a pessimist, but I see him more as a realist. This is a troll that knows life isn't all rainbows and cupcakes. He likes to be left alone. He doesn't sing, dance, or hug. And he most certainly does not high five.
The problem with Branch? No, it's not that he needs to lighten up. It's not that he is miserable and unhappy. The problem with Branch is that he is surrounded by extroverts who think he needs to be just like them. Sadly, the writers of this cutesy little film thought he needed to be like everyone. He was doomed from the first scratch of the pen. It's a sad end for poor Branch when the writers force him to completely change who he is, because of course he couldn't possibly be happy as he was. It's an introvert's nightmare of a happy ending.
But, I am not here today to preach about how we shouldn't use cute cartoon characters to reinforce negative stereotypes against introverts by shoving the message that you suck if you don't fit in and act like everyone else. However, I am here to tell you that you don't have to sing and dance to be happy. You don't have to want to be in the middle of the biggest, loudest, and craziest party ever to have fun. You don't have to hug everyone, even those you care about. And you sure as hell don't have to high-five.
I felt the greatest sense of empathy I have ever felt for an animated mythical creature in the scene where Branch was forced to high-five that annoying, pushy, boundary-crossing, asshole of a cloud. Branch didn't want to high five that jerk, but he was forced to. Forced into physical contact. Think about that sentence for a minute. I know most people will think my feelings on this are an overreaction (and honestly that is a big part of the problem for introverts) but no one should be forced into ANY type of physical contact. That included your high-five. I don't find high-fives to be fun. They are not a natural display of excitement, happiness, or accomplishment for me. I see them as stupid, pointless, unnecessary contact that adds nothing to my happiness or well-being.
Some people simply do not want to high-five. They aren't being stubborn. They aren't seeking attention. They don't want you to "encourage" them to take part. They legitimately have zero desire to slap their hand against yours. Respect that!