“I hate when that happens!”
Your boyfriend tells you a story about an awkward situation he had today at work. He was coming out of the elevator and said "goodbye" to a coworker, but then realized that they were both walking in the same direction. He didn't know what to do. You understand how he felt. You say this because you too have been in the same situation.
I hate when that happens!
This phrase means "I don't like that kind of situation."
A: I forgot my cell phone at home this morning.
B: Oh no. I hate it when that happens!
The word "hate" in English is perhaps not as negative as it might seem in other languages. In the example above, the speaker can say "I hate when that happens!" while smiling and laughing. Especially among younger people, saying "I hate ___" is common when talking about anything that they don't like very much.
You can talk about any situation with "I hate it when ___." For example:
I hate it when you send someone a text message and they don't respond for a day or two.