Shae is never a point of view character at any point during the books, so readers will generally agree that there wasnt much of an indication in ASOIAF that her and Tyrions relationship is one based on love and trust. Indeed, her betrayal at his trial is more of a surprise to Tyrion than it is to the reader. As much as he is cruel, Tywin was right; she was a whore, not his lover. He really shouldnt be all that surprised that she ended up speaking against him; she no doubt did it for gold as much as for her own survival. Game of Thrones, as a TV show, allows us to see a character in a less restricted fashion than when they are trapped between the pages. Its easy to see on the screen through Shaes jealous tending to Sansa (which does not occur in the books) and her frequent, unsolicited visits to her little lions chambers, that she does truly have feelings for Tyrion, even to the point that they blind her to the dangers of King's Landing. That her love is a detriment to her safety is something that Tyrion recognizes; Shae somewhat naïvely does not, thus leading to a rift in their relationship. That she uses his rejection of her to keep her safe as the fuel for her revenge is all the more heart-wrenchingly apparent in her testimony against him, not to mention resuming her profession as a sex worker and shacking up with his dad. The Seven Hells hath no fury like a whore scorned, right? Changing the questionable nature of their relationship in their books to one of love undoubtedly made Tyrions trial the most emotionally charged scene of season four, if not the whole series.