why do guys pick and tease girls if they have a girlfriend?

My coworker has a girlfriend that he claims to have but never talks much about her to our other coworkers. I;m guessing he doesn't like PDA. I have a boyfriend as well. We work at a pet Store and he's my manager, so he has to train me on couple of things and do a role play with me and see how well I do as a Sales Associate in educating a customers. Or he will talk to me about new products coming in and that i need to read up on that.

Anyways, every now and then he would always say that I am shy and fet me to talk and to talk to more customers and recommend a product.

Ever since me and my other coworker, Max and I always gets into a brawl and talking sh*t to each other. Tom wanted to join the fun. I would be mean to the both of them especially Max because he deserves it the most. Max is just an older guy around his 40s would likes to be annoying to all of his coworkers. Tom is in my age. I guess I started it since I would pick on Tom and Max that's why they like picking on me becuase of how I react to it, they think its funny, so they do it more. Anyways sometimes Tom would like to nudge me or trip me or stand in my way, so I tell him to go away. And he's like aww why you so mean....

Just yesterday we did a white elephant thing that we didn't get to do during december and he told one of my coworkers that he was gonna steal whatever gift I got. I got scratchers and he stole that from me.... like on purpose... because later he told me he didn't like playing scratchers... I think he just wanted to get me mad or something...

He never talks much about his girlfriend and it took him a long time to find a picture to claim that he has one. I just want to know why if a guy has a girlfriend, why do they like to tease another girl who has a boyfriend as well?

for the same reason females still flirt with guys when they have a bf. as humans we are highly social animals and require socialization as part of our "survival" if you will (strength in numbers right?) The form and execution may take different forms male vs female in how they do this - but it's the same concept, done in different ways.

just like males and females think differently, but we still think.
we communicate differently, but still communicate.

same with the need for additional and more socialization and the quest to know and become acquainted with more people (romantically or not). Lastly.. attraction is still attraction and just because you're with somebody doesn't mean your "i find that person attractive" gland turns off. And as humans, we can't help ourselves when we find somebody attractive - we'll still react to it the way we always react to it.

so.. in short.. its "human nature" be it male or female - we just react to it and handle it differently.

hmmmmm.... Its very interesting