Comfort levels, regrets

It's important to be comfortable, with someone or something.
But my question is, is getting comfortable a good thing or a harmful thing.
I can say that comfort is positive, it sounds positive, it feels positive, so how can it come off as anything negative? Well, people can get comfortable or accustomed to negative things. The way someone is treated, abused, or even in means of thinking. It's hard to admit to being a victim of comfort but it's even harder to leave a comfort zone. The familiar is always more popular than the unknown. I find myself being accustomed to the negatives and when I take a step back and realize that I am, it's pretty terrifying because if it hadn't been for that step back, I could have easily continued. Leaving the state of denial is the first step but now what. In things that I deem important, I am the type who will give it my all, go from plan a to plan z if I have to before I let it go.
Regretting something is probably one of my greatest fears. I don't want to look back at a certain aspect of my life and wonder what if. Regret is worse than a bad ending. When something bad happens, it happens and you can move on. Regret on the other hand, haunts you. When you aren't occupying your time, it sneaks up on you and fills your head with a million different scenarios and a million different endings that could have, would have or should have. I don't know where I'm going with this rant but I just felt like typing out my thoughts on this.