Because Yennefer was extraordinarily *****ed off at Geralt the first time she sent the "friend" letter since he disappeared for THREE YEARS and then when he needed a mage, went to TRISS first.
The second letter is either ironic or showing how mad she is about finding out that he's with Triss...AGAIN.
Yea, he feared Yen for fleeing from her, do you wonder why didn't he ask her first? I don't...
It was not ironic, she was pissed off again... Man, but seriously NOONE cares about his amnesia?
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Geralt was also quite a dick to be honest. He was too coward to ask Yennefer directly, so he wrote Triss instead. Knowing that she had developed more than just a friendly feeling for him, but it was easier for him to decline Triss' advances, play dumb and hurt her feelings than facing Yennefer.
Triss even wonders why he asked her and not Yennefer, making her even some hopes, just to crush them a bit later...not cool Geralt.
Exactly, it was really cruel of Geralt to give that hope to Triss. He didn't allow her anything after that kiss at the stables, Triss must have suffered a lot... He caused both Triss and Yen pain, he isn't really good person there...
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If someone is interested about the conversation between Triss and Geralt from the Blood of Elves you can read it here:
Man that dialogue shows SO much about their relationship. Everything, she loves him SO MUCH and he just CAN'T return it. I think the saddest part of all books is that part at Kaer Morhen with Triss...
