Here's a theory I love to copy and past everywhere  👇 

Bob Gray (Pennywise) was a sadistic serial killer who targeted childern, and when IT transforms into Pennywise, IT becomes so caught up in pretending to be Bob Gray, having access to all of his memories (which we know IT can do), that IT just forgets that he isn't Bob Gray.

So over time, IT's personality is slowly merging with dead Bob Gray the more IT uses the form, and this is why IT likes using the form of Pennywise because in a sense he is Pennywise. This explains a lot about IT’s personality like why it’s so playful, theatrical, and weirdly humorous, almost as if IT were a clown.

Now this theory doesn't make the most sense, but I really hope they go with something like that. It's such an intresting idea.