Why is he nervous around me?

So I have a friends with benefits. We go to the same school, but rarely see each other because we are in different departments. When I ran into him last week, I could tell he had to build his confidence to come say hello. My friend said she has seen him do this to me before at school, parties, etc. This was my first time noticing it. Why is he nervous? Does he like me? Does he not want to be seen in public with me?
I don't understand how someone can be nervous when they have been sexual with the person several times.
Why is he nervous around me?
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