A Tower Local Controller reported an aircraft on one half mile final initiated a go around because the pilot felt it was too close to an aircraft on takeoff roll.

Narrative

I put Aircraft X in position after a departure and with Aircraft Y on final. I cleared Aircraft X for an expedited take off with what I considered sufficient time and spacing with Aircraft Y on final and the departing aircraft did not start their departure roll as expected. I then reiterated to Aircraft X to expedite their departure and their company was within a mile. Aircraft Y initiated a go around on their own outside of a half mile as Aircraft X was already on departure roll and a critical stage of flight. I subsequently asked the pilot of Aircraft Y the reason for go around and they indicated that they thought it was too close to the departure for their comfort. I felt that I would have had adequate runway separation if the pilot would not have initiated the go around considering the arriving aircraft slowing down and the departure accelerating. I take this event as a learning lesson and contribute expectancy bias to be the main contributing factor on my part as the controller. I will implement more consideration for spacing and timing and aircraft characteristics and performance.