Why would Geralt, as a person, end up with Yen, as a person? Let me start off by saying that Sapkowski was quite obviously a man who understood human character when writing Witcher books.
Geralt wouldn't do anything for Ciri. He's too moral, too idealistic and too easily swayed by emotion, he lacks necessary insight and foresight to protect her completely. He's a great father figure, but he's not a great protector. There's one thing Geralt knows he can depend on tho, and that thing is that where his morals would stay his hand, Yen would step in and heartlessly do whatever needs to be done. Yennefer is cold, rational and extremely powerful. Yennefer also loves Geralt and Ciri unconditionally. She will lie, manipulate, murder or even steer fates of entire kingdoms if this would result in protecting the two. In short, Yennefer can do a lot of things Geralt can't, and she's not afraid of making use of them. Geralt also knows that he can trust her. Yen won't lie to him - she won't always reveal the whole truth, but he knows that whatever is it she's doing, it will end up helping the trio in one way or another. In my opinion, that's what you look for in a partner - you look for a person who will fill in the blanks in your personality and help you in areas you're not good at.
And then there's Triss. And Triss is, to put it bluntly, weak and selfish. She has aspirations beyond Geralt and Ciri and if they stand in her way, she would abandon them, sooner or later. After events of the books and the games, Geralt has no real reason of trusting her.
Albarok: And the constant belittling and disrespect is fine with you too?
Oh they actually respect each other massively, albeit it's apparent who wears pants in that relationship. Still, Geralt always makes sure to come up with healthy retort, and relationship without constantly teasing each other is just boring :-P