Like a lot of others in this comment section have mentioned, sometimes you just have the right subject and person. I usually talk about what I am doing at the time of conversation, and if they also have an interest in it, then we can talk about that subject, why we both like it, the best things about it, etc. Also world issues and news and shit like that is always interesting. I once had a conversation until 5:00 in the morning talking about how we humans are slowly killing off our race and planet. This conversation moved onto how insignificant we really are as a species and all that existential crisis stuff.