Why is he so insecure?

I am talking about my boyfriend whom I've been in a relationship with for over 5 years. To celebrate our 5th anniversary, we booked a trip to SA and Namibia recently. In Cape Town, it was pretty hot for us and I donned my bikini which looks analogous to the one below worn by the famous travel host Alex Outhwaite (I bought it inspired by her looks). My boyfriend wasn't happy when a few guys checked me out. I didn't encourage their behaviour and went away from them. At the second spot, both of us saw a teen having a tent after staring at me once or twice. Albeit I changed my spot again, my boyfriend wasn't happy about those guys' actions. I asked him not to worry since I love him and only him, and wouldn't pay heed to others' advances towards me. I even held his hands tighter to assure him. He's still upset about it.

Could you please help me?

Why is he so insecure?
Why is he so insecure?
Why is he so insecure?

I mean this looks so good on Alex and everyone who wears it!

Why is he so insecure?
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