Human beings are a collection of personal perceptions over their experiences throughout the history of their lives.
Anyone who spends a lot of time doing something expects it to remain the way they've done it, since it always has. It becomes a part of your identity, intentionally or not. Even if it's only a small portion of who you consider yourself to be, it's been woven into your life for years, decades in some cases, and shaped your sense of self and position in reality in whatever myriad minor ways that it has.
Whether that's lore mastery, political ideology, religious belief, relationships, sports teams, trivia, or job training. The second people challenge that aspect of yourself, no matter how trivial, there's an inherent instinct to defend that aspect of the identity you've created for yourself.
Some things drift. Some things change. People slip into different beliefs as new experiences change them, and sometimes have to let go of the past in an incredibly painful way because when you leave that thing behind it has still shaped you, still left it's mark.
There's still a food that you love because someone who is no longer in your life introduced you to it. A Musical Style or artist. A specific laugh. A particular time of day to sip on a hot drink and look out at the world. Because someone who is gone from your life made that a part of who you are. The smell of a specific scent will make you think of them, and your heart will ache for a moment, and you'll move on.
For people like us, for whom D&D is an aspect of our identity, seeing that relationship change can be just as heartbreaking as losing someone we love.
Thanks, Henadic.