Healthy Blood Through Rest: Introduction

In my early twenties, I was experiencing bouts of tachycardia. I went to the doctor for a diagnosis. He noted that yes my heart was functioning in an abnormal manner and suggested about 10 different tests to figure out the cause. He told me that if he could find nothing after the testing, it was probably just stress. I was confused why he didn’t just try to rule out stress first. Why was it at the bottom of the list?