Why lie?

The best lies are based on the
truth, at least in part

Lies and secrets, Tessa, they are like a cancer in the soul. They eat away what is good and leave only destruction behind.

He touched her face, gently, with the back of his hand. 'you disappear so completely into your head sometimes.' he said. 'i wish i could follow you.'
you do, she wanted to say, you live in my head all the time

You could have fooled me. Everytime I called you, Luke said you were sick. I figured you were avoiding me. Again."
"I wasn't. I did want to talk to you. I've been thinking about you all the time."
"I've been thinking about you, too."
"I really was sick. I swear. I almost died back there on the ship, you know."
"I know. Everytime you almost die, I almost die myself.

You guessed? You must have been pretty sure, considering you could have killed me."
"I was ninety percent sure."
"I see," Clary said. There must have been something in her voice, because he turned to look at her. Her hand cracked across his face, a slap that rocked him back on his heels. He put his hands on his cheek, more in surprise than pain.
"What the hell was that for?"
"The other ten percent.