Bullish sentiment is something that can be erased very very quickly. I remember why 2018 was such a bad year, because when we reached peak point at November 2017 there were tons of whales who ended up selling, those were the days most of the early investors got out, all those people who bought under 30 dollars started to sell, I am sure some of them are still around but a lot of them sold and considering they were capable of selling thousands of bitcoin easily (because they made so much bitcoin on early days even with mining) that became something a bit more different, where people would come out and sell tens of thousands of bitcoin.
This time around we do not have that just yet, so it looks like its bullish right now, but the moment someone comes out and sells tens of thousands of bitcoin, everyone else will start to sell their bitcoin as well.
Definitely this. BTC is just one whale away from dumping to 29k with that whale selling only a few hundred of their thousands of coins. This rally will see some real pull back when a whale decides to sell, and I would think one will soon as with a new year comes a new tax season.
Will buyers (and institutions, and grayscale) be able to absorb the selling? I'm optimistic they can, but it wouldn't be BTC if we don't revisit 20K with some random selling at 3:42AM across every exchange. When it happens, you know some person just became a billionaire in fiat terms. Have fun with the taxes!