why did he get so mad?

So I went up the to back evict of a bar last night to go talk to my friend and catch up, the guy who I used to work with was up there, he attempted to shake my hand but I rejected it because he was so immature and disrespectful when we worked together, and we are both in our 20's, now I can understand feeling some type of way about it, but the guy got really mad and made a big deal about it, as if we were friends.

Nuff Sed2016-02-06T08:44:50Z

People often get mad when they're insulted in public, especially if it's a surprise and they expected better behavior. Refusing to shake can be interpreted as a personal repudiation, somewhat like spitting on them.