Why does he get so jealous easily?

At first I thought it was cute that he got jealous a lot but after a while, it's just gotten to a point where it's really annoying now. Why is my boyfriend like this? I don't think I've ever given him a reason to doubt his trust in me. To top it all off, we just had our worst fight ever last night.

I was hired as a waitress for a foreign socials, while he got hired as one of the chefs. One of the guests got drunk and started hitting on me, to which I promptly shut him down. Despite this, he still got mad at me and stormed off after the event. I'm so sick of dealing with this and just want to understand why he's acting this way.