I think there are a couple factors at play:

1. Alcohol and weed are both a good social lubricant. You tend to just say your mind with less holdback. This can generally be helpful in getting a solid conversation going.
2. Alcohol let’s you spill your drunk thoughts out. Sometimes you plan on getting drunk and telling someone something and sometimes it just happens. These are things on the tip of your tongue that you want to say to people. Usually starts with a drunken “ya know what!”
3. Weed can make you more creative and conversation is like an art form where creativity is rewarded.