Being complacent, thinking that someone always has your back and that you do not need to worry on matters that you should be concern with is, as you are already aware, never good. Despite its many negative social repercussions, the main point I would like to reiterate here is that it hinders progress. And to be successful in life, achieving a continuous progress along your journey is very important.
Yes, I admit that it is not easy to not have that emotion creep up in your heart. However, that is where contentment comes in. Be content with whatever you have but do not be complacent.