If you were mad at a colleague for multiple decades in governance (Pelosi) pulling a Brutus live on Morning Joe and giving everyone tacit approval to knife you the requisite 23 times, because some donors were getting spooked, and it was all at the word of that mulatto shithead you spent years taking heat for who STILL didn't back you in 2016 OR 2020, and then, on top of it, they force the nice young lady you picked to put a relatable face on your campaign and carry on your legacy to distance herself from you, wouldn't you be gleeful when they get shit on?
Joe is also obsessed with attention, even moreso than a standard politician. Like the only time he and Harris ever came to any kind of conflict in the text is over him endorsing her in his withdrawal speech, because he wanted to hold off until the following week to endorse her so he could be the top headline over the weekend. She had to remind him "Joe if I'm not in your speech Obama is going to get his way and have his open primary instead of me being able to smoothly step up", and THEN he agreed to endorse her faster, in a separate speech, the afternoon after the morning he signed the letter.
(side note: I forgot to mention it, but I do totally believe Harris genuinely likes Biden, or at least feels extremely indebted to him, it's not the writers trying to whitewash her reputation. There were many times where her staffers were begging her to drop him, it was the politically advantageous thing to do, and the authors are actually criticizing her for not doing it, it's practically the only thing they do criticize, but she doesn't)