I had a colleague at work who was absolutely fine, but one day started to hate me. Wouldn't even say good morning.

This went on for years, and then she eventually ended up working in my team. We had to communicate a lot, and she realised I wasn't as much of a dick as she thought. Once we became friends she told me the reason why..... I had explained something to her that day, many years ago, with a stupid condescending grin on my face and she didn't like it.

I mean, dude, that's just me trying to smile and be friendly :/