“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, because you've had situations like this before as well. You say:

I hate when that happens!

I hate when that happens.

This phrase means "I don't like that kind of situation."

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.