Shae goes through a pretty drastic change of heart in the middle of Game of Thrones season four, and we only glimpse the reasons for it. But here's a deleted scene from the second episode of the season, where Bronn gives Shae some advice that doesn't work out that well for her... or for Tyrion. Some spoilers below...
This scene gives some great insight into Bronn's character — he knows that sellswords have an expiration date, and "you never see an old sellsword." He can't just keep fighting against younger men, he has to learn some new tricks. And he encourages Shae to do the same thing, since sex workers similarly have an expiration date. And that advice may contribute to the decisions Shae makes later in the season, which could be her version of "learning some new tricks."
Here's a second deleted scene, from episode eight, which mostly just shows Daenerys growing more paranoid: