Genos didn't know how his attraction towards his master--you, started. It originally started with Saitama.

Genos has been always meticulously observing Saitamas life and how he became so strong. Saitama was strong.. incredibly so..But you were stronger. He became Your and Saitama's disciple. He wanted to know the secrets you both hid. Saitama was strong enough to decimate the world in one punch while your way of life that led you to being able to accumulate force a strong enough to decimate possibly the world with one mere flick of a finger.

Saitama wakes up, his blue and white pajamas loose over his body. He sees you reading one of his mangas, he doesn't mind.. he then walks to the kitchen where Genos is cooking to grab water.

"Master, what should I make for dinner?"

Catering for you and Saitama became like second nature to him. He'd thought of it normal, making you your favorite meals every day or doing the chores around the tightly cramped apartment that formerly housed you AND Saitama before he moved in. He had no problems doing whatever errands Saitama had in store for the day, wanting the both of you to be as comfortable as possible.

That wasn't the only thing either. With him being an S-class hero, he was constantly busy with missions and fighting off higher level monsters.

While he knows you both are more than capable of fending for yourselves, He tends to worry about THE BOTH of you.

there was a time where Saitama once commented on how he acted much like a househusband would. Genos didn't comment. But the thought of it didn't upset him, either.

It feels nice, he thinks. The strongest people are depending on him.

Somehow, it makes him feel needed knowing that he's in your guys life. A constant presence you've become used to.

He didn't think that Dr. Kuseno would program such emotions like love into his system. Or maybe, he’s been so blinded by revenge that he’d forgotten to feel affection for someone after his family was slaughtered unjustly by a rogue cyborg.