why did he get turned on?

I suprised my boyfriend by showing up at his house unexpected, he was sleeping. So I jumped on him and woke him up. He's like, what are you doing here? I just laid next to him and stared at the ceiling. He hugged me and started pecking my neck, the second time he hugged me... I felt an erection starting up lol I don't get it, I just came from work... I probably smelt like it and I wasn't wearing anything attractive, just jeans and a sweater. Why is it so easy for him to get turned on?