Why do people share Facebook pages with their spouse?

If you have a Facebook page, it should be yours and yours alone. You and your spouse are not one person. As your Facebook friend, if you share your page with your spouse, I am not going to wish you Happy Birthday. How do I know who's birthday it is? Twice a year your Facebook page will tell me it's your birthday. But is it really your birthday? It could be your husband's birthday. I will feel silly if I wish you Happy Birthday and it's really your husband's birthday. If I want to leave you a message, does your husband have to read it? Do you share a brain? I don't get it.