It's the same for me. I can have a short stutter free conversation with someone I don't know and don't care about, but speaking with my mom is hard. that's how I explain it: When talking to someone close, you care what they think of you, so you are invested in the conversation. When telling a long story you get nervous because you are again, invested in it, and I suppose you are not used to talking for such long periods of time. Arguments are tricky because you have to process a lot of information quickly, builds a lot of pressure. tl,dr: being invested in smth/somebody or pressure is stress and stress = stutter.