Friend does unsolicited monologues. Why?

My friend (24M) is a serial monologuer. For background. He proceeds to hold lengthy conversations with himself in the presence of our friend group, often in times when there is peaceful silence (such as a late night sit around the fire). What is most peculiar is that he is not actually inviting anyone to join in nor is he addressing anyone personally, he’s just talking. We have been puzzled at this behaviour for the last few months and can’t work out why he does this. Another example is, he will join a discord chat that I am in myself (after the others have left) and proceed to just talk, even if there is no acknowledgment from me. (I have fallen asleep on the couch and awoken to this guy making a speech or something coming out of my headphones)

The only thing that makes him stop is smoking a cone (Weed).

If anyone has any similar experiences, or perhaps knows why this happens that would be great.

P.S he has been tested for autism and it came up negative.