It's wrong to take your friend's beer.
Jake and Bill are at the bar and are talking over some drinks, Jakes cup of beer is empty, Bill's is not.
Jake wants to drink more beer.
Jake calls the bartender over and orders himself another glass of beer.
Jake grabs Bill's cup and takes a big swig of Bill's beer without asking Bill first.
Bill continues to talk with Jake and the two of them have a fun and happy evening together.
Bill is angry that Jake is spreading his germs all over his cup and taking his beer without even asking.