Ugh, I've seen this come up so much in the web novels I follow. I think it often starts with the quiet, unspoken things that aren't yours. You walk into a house where every photo frame, every piece of furniture, every inside joke belongs to a history you weren't part of. They might be perfectly nice, but you're an add-on, a guest star in their long-running series. There's a rhythm to their days—how they celebrate, argue, or even just sit in silence—that you have to learn from scratch while they've had decades to perfect it. That initial politeness can sometimes be the worst part, because it means you're still on the outside of the real, messy, comfortable family dynamics.
Another huge factor is loyalty. His primary loyalty is, naturally, to the family he grew up in. When there's a clash of opinions or habits, even if he tries to be fair, you can feel him being pulled back by this invisible cord to his parents or siblings. Suddenly, what should be a simple decision about a holiday visit becomes a silent referendum on where his true allegiance lies. You become the 'new variable' that's disrupting their established system, and that pressure to adapt is entirely on your shoulders. The loneliness doesn't always come from malice; it comes from being the only one who has to consciously translate everything.