Have you ever wanted to make sure everyone knew what was yours, so they'd keep their hands off? Well, if you haven't before, you're feeling that now, and those emotions are not towards a possession. They're towards a person you want to claim, no matter if your personality is territorial or not. Or maybe you're the one wanting to be claimed. Either way, this will end in sex that's either public, running a high risk of discovery and publicity, or that leaves marks
There are as many reasons why this desire has cropped up as the imagination allows: frustration, sexual tension, disbelief of others that you're a couple, anger at others trying to steal what's yours, punishment or to remind someone they're yours, a personality-altering gas, hormones, heat, a mutual kink, for fun, for showing the world how much you love them, or even in order to save them and show some despots that this person is taken because better you than anyone else. How serious the "claiming" instinct also depends on entirely upon what you really want.
Remember, wanting something to be your own isn't bad.
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