I've thought about bullying a lot. Here are my elements of bullying so far:

bully wants something he (or she, of course) is not entitled to

bully objects to something that is not really a problem

bully charges in fast, too fast for you to collect your thoughts and defend yourself

bully often is a bigger person, is used to dominating others with his size

bully is too loud

kids may bully outright to be mean, but adults learn to cloak their bullying in claims of being more proper, honorable, discreet, etc.

bully seems to know unspoken behavioral standards that other people don't know about

bully stands too close, invades your space

bully implies that other people are ridiculing you behind your back

bully may be conscious of the audience, making you look inefficient in front of the boss or making you look foolish in the presence of that certain pretty girl

if you come back at him with a good defense, the bully suddenly has more important business elsewhere

bully leaves fast, often slamming the door

I suspect a lot of people are bullies in their teens and eventually improve. Either their minds mature or they wise up and realize that they have no friends and no future. My brother was a bully, but he changed when he topped out at 5'6".
===============
P.G. Wodehouse made a brilliant portrait of a bully when he invented Lady Constance Keeble of Blandings Castle.