get under somebody's skin
get under (one's) skin
1. To become a source of irritation. I just can't stand Dean's voice, so everything he says gets under my skin.
2. To be a source of obsession. I can't get that beautiful girl I met earlier off my mind. She's just gotten under my skin.
3. To gain a rich, thorough understanding of one. A: "How did you know he would make that decision?" B: "Oh, I feel like I've gotten under his skin since we've been spending so much time together lately."