There's no perfect script for how a physical relationship should unfold. That means any interaction which might turn sexy can be fraught with ambiguities. As people, sometimes we're just not looking for those signals right away. That or we don't believe anyone would ever be as direct but also indirect as "can you help me change my sheets with me in them." Whether it's because people are scared, overly polite or totally oblivious, there are times guys miss obvious signals.