Part of it probably has to do with his hatred of short sellers. People who short sell have a vested interest in seeing the stock they've shorted drop. They tend to make their opinions known and attempt to create a self-fulfilling prophecy.
"I bet a billion dollars this stock will go down."
"Well shit. if he bet a billion dollars the stock will go down I better sell it before it does."
Gee the stock went down!
People have been vocal about shorting Tesla. Musk probably sees their bets against his company as a personal insult.
GME is massively short sold. By some accounts many multiples of existing shares have been sold. I'm not sure that's legal but it seems like it's being done.
If a bunch of people buy and hold a massively short sold stock the short sellers (who've sold a stock they don't own) have to buy it back to cover their promises. This causes a short squeeze which spikes the price and costs the sort sellers massive amount of money.
https://www.cnn.com/2021/01/31/investing/melvin-capital-redd...
Or maybe he just likes the stocks. There's an argument to be made that Gamestop is undervalued. That AMC will emerge from the COVID crisis stronger than ever and that Dogecoin while it seems like a joke is actually a cryptographically and structurally sound cryptocurrency.
Although probably not the latter since he claims he doesn't own stock in any publicly traded company except Tesla.
I think he's just in it for the memes.