You shouldn't access someone else's online accounts without permission.
John hears that Dave's girlfriend broke up with him by sending Dave an email.
John wants to find out if the rumour is true.
John calls Dave and just asks him if it's true that he had been broken up with through email.
John hacks into Dave's email by figuring out his birthday is his email password, so he can read the breakup email.
Dave is happy John is taking an interest in his personal life and is happy to share the breakup details with John.
Dave finds out John violated his privacy and is infuriated and feels violated and distrustful towards internet security.