Aside from the fact that people like whole numbers, I think the whole coiner “movement” (if we call it that) yields a lot of psychological security. When people reach this milestone, they feel good that they are part of an exclusive club because they know that they are still in the minority and that the majority of Bitcoiners have not reached this target yet. Other people may value it as a form of bragging rights to seek validation from the people around them that they are good with money, disciplined investors etc