But you might not want to. Reasons you might use POST instead of GET include:
The total amount of characters in a GET is really limited (depending on the server). If the user types, say, a long passage into a “search” input box, the GET might not work.
The data you send with the GET is appended to the URL up in the browser bar, so whatever you send is exposed. Better not put a password or some other sensitive data as part of a GET!
Because of number two above, the user can’t bookmark a form submission if you use POST instead of GET. Depending on your app, you may or may not want users to be able to bookmark the resulting request from a form submission.