Even if it was possible to feel at ease and not in a work mindset while hanging with them, it's just wise not to get that close, it makes it harder in future to be selective in what you divulge about your private life which can give your boss leverage over you. Sometimes you may need a "sick" day and it's just better if they don't know enough personal information to be able to determine how sick you are and make everything awkward.

That might seem dishonest, but there's reasons why you might need to the employer to know only what they need to know and they aren't necessarily laziness or incompetence. It's a shame because it's nice that your boss wanted to be friends but unfortunately there's always going to be that fact that they're your boss which gets in the way of that and everyone is better off keeping things arm's length