Why does he get so hard?

I've been hanging out with this guy since the beginning of the school year. I'm a senior and he's a junior. But I see him during lunch all the time and we hang out instead of going to eat. Well we've been flirting a whole lot since school started and I really really like him. And I know he likes me.
But lately he's been more flirty and always makes excuses to touch me in some way. Like when we go down the stairs he'll grab my hand and says he's afraid I'll fall. And he's always tickling me or hugging me now. Well lately when he does that I can feel his erection against me. And yes it turns me on a lot. But why is he getting hard jist from a hug or touching me. And sometimes I've noticed him getting them as soon as we start walking together.
So is he attracted to me... and if he keeps getting erections should I say or do something? And I've noticed I get really wet around him as well...
Why does he get so hard?
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