Why so nervous about putting in a two weeks notice for a company and team that’s lacking?

Long story short, I just got offered a job that is a step up from where I currently am, but I’m nervous about putting in my two weeks tomorrow. I feel as though the company I’m currently with is taking advantage of me, and my abilities. The only coworker I enjoyed working with, left the company a week ago and I’ve been miserable. So I started looking for better jobs and found one, but why am I so nervous about putting in the two weeks notice.