People apologising for normal conversations they had while they were drinking?

This has happened to me several times and I don't really get it:

I will be at a pub (or somewhere else where alcohol is being served) and get into a nice conversation with someone I don't know very well. I'm pleased that they are starting to open up to me and I am getting to know them a bit better.

There is nothing odd or offensive about anything they say.

The next time I see them (or they run into someone who can relay a message to me), they apologise for their behaviour and say that they were drunk and hope I wasn't too offended.

It kind of bothers me because there I was thinking that we were getting to know one another better and now the other person wants to pretend that the conversation never happened.

Has this happened to anyone else?  Can anyone explain why people here do this?

Two potential reasons I can think of:
1) They don't remember exactly what they said, so best to apologise in case they said something offensive.
2) It's not considered polite to talk about some very personal topics with someone you don't know, and people here can often find that an uncomfortable situation. They're apologising in case they made you uncomfortable with the level of openness they had while drunk. IME Americans tend to be far more open about personal topics even to complete strangers, whereas Brits are more reserved about sharing certain things.
I think if this happens again, just say something like, 'I wasn't offended at all, I quite enjoyed our chat.' Unless they really did say something offensive or something very personal that they wouldn't want repeated, I imagine it's more that they're trying to be polite and wanting to avoid YOUR discomfort rather than wanting to pretend like the conversation never happened for their own sake.