Why did my crush walk away laughing when i rejected him...?

Ok there was this i have a huge crush on since 2 years ago. He only noticed me in a class last semester in college and i kept giving him really obvious signs that i am into him (constant stares, looks etc...) and he replied the same way too but sometimes gave signs of disinterest. He never made an approach till near the end of this semester when he came up to me and asked me out while i was in a food area with a friend with his two friends standing at a near distant behind him. I got nervous and mad at the same time he took this long so i blew him off in a somewhat cold manner in a crowded area, didn't even give him eye contact after i saw him walking toward me. He walked away with a huge smile/laugh on his face. WHY would a guy do this?