I'm on really good terms with my colleague, we go for lunch every day, we run our department and we communicate efficiently. However, he keeps telling me about xy all the time and I kind of am starting to get a bit tired of it. It's a really pleasant subject to him and matters a lot to him.
Also I know I'm not good at being sensitive so I don't want to risk coming across as rude when I would ask that perhaps we could talk about something else.
How can I say this politely?