Why do I keep having dreams about people in my life dying?

Many things. There is not enough space here to really go into it but these dreams seem to bely some anxiety about abandonment, lack of conviction of self reliance, lack of confidence etc. If you have a therapist i would talk to them about it, if you don't have one...Get one.
Don't focus on it. I recommend reading on mindfulness. It might be very helpful in your dealing with these dreams and the stress it causes.