Why are people mean dude

Why are people mean?? Like for no reason. today this kid yelled at me because I like couldn't find my linoleum block, but he told me where it is while yelling so I said thank you. and he said "thank you 🤓 " in a weird baby voice?? like tf?? and then I asked him why he said it like that so I was like "why do you say these things? I'm literally just saying normal words" He was like "can you stop talking to me?" and he plugged his ears. then later on His friend whispered something to him and he responded with "why are you saying these things I'm saying normal words!" (in mockery of me of course) and then he saw me looking at him because of this and said "ohhoho sorry my bad" (but in a way that I could tell he clearly didn't mean any of what he said) I ended up walking out of the room and crying. Im not sure why this was so upsetting to me because he's said meaner things and he's never really gotten to me like that? but my point is that he has no reason to be so rude. even if he has issues at home or something, bullying others isn't going to fix that. 

the reason he started bullying me in the first place was because I was friends with this kid that he already bullied so for him it was like 'hahah! new victims!' and he's also now bullying my GF because we obviously hang out.