I've no idea why people do this or what they are hoping to achieve from it. I just find it really rude and belligerent.
I've got people in my life who don't believe anything I say. They say it over the most stupid things and I'm not known as a liar.
Eg Do you shower every day?
Yes, I tend to shower every day
Even if you're not going out?
Yes
I don't believe you!
WTAF? How does this person know I don't? Who do they think they are to say they don't believe me? It's ridiculous. What are they getting from this? I'd never say that to someone, even if I didn't believe them.
What am I meant to do when someone says they don't believe me?