One of the things that really gets me about Letho is that he has huge amounts of respect for Geralt and not only because he helped save his life that one time; mostly because Geralt seems more human to Letho than he ever sees himself becoming. He has friends besides other witchers, a family of sorts, a place to return to each year and call home, the ability to blend in with ‘normal’ people and form meaningful connections. All of these are things Letho either never had or he had and lost them forever. And sure, no witcher will ever be fully accepted among humans as one of their own, but Letho is barely accepted among witchers, let alone common people.
It’s part of the reasons why he helps Geralt time and again-- taking care of Yennefer, saving Triss from imprisonment, helping fight the Wild Hunt for Ciri. He’s not just helping Geralt, he’s taking care of those who are close to him in one way or another, be it lovers or friends or adopted daughters. Because he understands how precious and rare this is for a witcher and he’s willing to put his life on the line to ensure that at least one of them gets to have a family and friends.
And idk man. It just makes me emotional. All his fuck ups are exactly because he’s so lonely and longing for all these connections he doesn’t even know how to create and preserve. He takes on the assassinations jobs because he wants a home and to see his Viper brothers reunite in one place where they can be safe. He trusts Louis because he was willing to hold a pleasant conversation with him. He’s so desperate for an ounce of companionship that it will override any rational thought and self-preservation instinct.