Yeah, I’d say SO generally are like great friends (with benefits), but often, over time you end up doing so much stuff with them that they eclipse any other individual friend. And eventually (especially if you end up living together) they become such a part of your day to day life they can begin to feel like an extension of yourself (or rather, that you’re both part of one being) and it’s hard to live with out them. I remember laughing at my dad for wanting to phone my mum everyday when he was away on business. I’d be like “what do you even have to talk about? You see each other all the time” But now, if I had a day where I didn’t at least message with my partner I’d feel so isolated.