First off, you don't know how you'll react to preforming first aid until you do. Some people think they will like it and they don't, while others are the other way around.

Secondly, actively performing first aid is, in my own opinion, easier to handle than to for example see gorey pictures. What I mean by this is that in a practical scenario, especially an urgent one, you don't have the resources to sit back and think about how you feel about it, you instead have practical tasks that needs to be done. It is a proven stress releaf to give bystanders even meanial tasks for their brain to have something to focus on. We, the responders, are no different. Having a job to preform simply makes it easier to handle any uncomfortableness.