The first time it happened I was standing still for awhile when I received a text message that my HomePod had been delivered to my doorstep on release day. I was initially concerned about my elevated heart rate, since this had never happened before. After some thought (and no chest pains or palpitations) I concluded it was the good news about the HomePod delivery that spiked my pulse.
The second time it happened I was halfway through a flight from San Jose to San Diego. I was reading several articles where Jony Ive was detailing the excitement he feels when tackling new problems as he explained his design process and philosophy. While reading his words, I could sense his excitement without realizing it and my Apple Watch alerted me to my spiked heart rate. I quickly reached the same conclusion as the previous time, plus, I tend to have a very high max heart rate that's over 200 bpm.
I guess I truly and deeply get excited over Apple. I'm sure there are others.