My theory is that people buy it as holiday gifts.
Which December? It has just raised only from 3K to 4K! It is not enough for us. It has to rise more and more. Most of us still in loss! This December is not going well... We still are waiting.
I think this increase will not continue. it's still possible that bitcoin will fall deeper again. so keep watching the market movements and don't be greedy if you have made a profit .. .. 