To explain this I have to appear very shallow. Behind every person lies a number, their rating, if I may. Let's just say, for the sake of argument that this number is between 1 and 10. Obviously, no one is the same rating exactly, but I'm going to explain it easily because I'm lazy. If a boy and a girl know each other and the guy is a 6 and the girl is an 8 (I hate the way that sentence is worded but I can't find another way), then the guy will pretty much always desire the girl, because she is two "points" above him.
All human beings are intrinsically shallow, whether they admit it or not. Upon seeing an amazingly attractive female, all men will lust (sorry to ruin your shroud of ignorance). It is also true the other way around, but it mostly applies to males.
So before you see your friend as just a friend with no complications, consider the possibility of him wanting more out of the relationship, regardless of what he's actually told you in person.