Because they're being busybody assholes, that's why. These are the same people who will start bugging you about popping out 100 babies during the first minute of your wedding reception. Really, you should feel happy that you're doing interesting things with your life and have better things to do than worry about when anybody else is getting engaged -- and sad for the people who have nothing better to do than ask you invasive questions about your personal life.
When I get married can I wear a stand up for choice pin on my wedding gown?