Is sexual tension always mutual? Is it obvioys to others? What are signs? Why am I feeling it with this guy out of nowhere?

Anonymous
In 9th grade, this guy ( "D" ) & I always had a flirty vibe but he ended up getting a girlfriend at the end of the year. I liked him for a minute but I got over it. This year (junior yr) we have classes together & we talk & are playful sometimes nothing unusual.

The other day I had to sit next to (D) to work on a partner assignment and he sat very close to me.( i could feel his leg sweat on mine at the end of class thats how close). We were doing our work and joking /teasing each other but I felt a lot of sexual tension..
At one point he kept whispering in my ear about some funny and we started laughing. His friend said something to him about me not being his girlfriend.. he ignored him and stared at me and we worked on the assignment. I dont know what his friend told him that. It was a bit odd that he was sitting so close but the sexual tension is what surprised me.
Is sexual tension always mutual? Is it obvioys to others? What are signs? Why am I feeling it with this guy out of nowhere?
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