I sometimes purposefully interrupt people so they lose their train of thought and talk about something else.
Why would you mess with someone in the middle of expressing something?
I don't like it when someone discusses something I find stupid or boring.
I can sympathize with the fact that you find some discussions boring, but messing with someone to draw them of topic is incorrect. Handle this by suggesting a new topic that you can both agree upon instead.
I just can't. I don't feel bad. I'm happy they are distracted by my foolishness.
It's expected that you have a certain set of manners when you are around others. It's just incredibly rude of you to interrupt others when they are speaking.