Why does my heart jump every time I get a text off him?

I've been texting a guy on and off for about 4 months now. We just literally talk and have a bit of banter. I know he doesn't want a relationship as he's told me this, and I don't want one either. He's told me her literally wants to be FWB. I don't fancy him but whenever he texts me out of the blue, and I see his name come up, my heart literally jumps out of my chest... Why? Has anyone had this experience before?
Why does my heart jump every time I get a text off him?
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