creepy guy at college

There's this guy at college i don't know what his name is he was with me in a class last semester but i completely ignored him and so did a few other people in my class.
I see him at college and i pretend that i'm busy with something so he leaves me alone but no he has to come up to me and talk.  it was really awkward
Later i head to the lab to get some work done a few minutes later i feel someone standing next to me i look up and i saw that it was him i felt like i just wanted to disappear into my seat.
He then starts talking to me about god knows what.  Thankfully my friend comes but before she doses he asked me for my number i felt my heart stop.
Why would he ask me for my number? i don't know who he is. he shouldn't ask for it.
I said no, he tried to convince me to give it to him i'm like sorry i cant i walk up and leave.
My friend gives me this weird look and asks me what happened so i told and then she says " its always the shorts ones"
i don't mean to be rude or anything but yes he was short and creepy.
My question is: why do people think that just because you spoke to them gives them the permission to ask fot your number and why is it that when you say no they still keep on nagging you for it?