Why do I have to enter a security code on every login to read e-mail?

I have had two-factor authentication enabled for a while. For the past few days I have had to enter a security code sent to my phone every time I log in. Each time I make sure the box that says not to ask me again in this browser is checked, but invariably I have to do the same thing on the next login. I have just disabled two-factor authentication, because it has become too annoying, but that is certainly not my preferred solution!