Most Helpful Guy
Because we get a lot arousal from visual/audio. So a girl looking hot and talking about wanting sex is a super turn on.
Also Since you've been together before he will be remembering/imaging it again.
Young BOYS can get an erection if the wind blows in the right direction. As we get older we usually need more physical/talking dirty... but I still get aroused much faster than the mid 20 girls I've been with.
Most Helpful Girl
Because them getting horny is independent of when you decide to unzip their pants and pull their dick out.